Your first experience notwithstanding, (we'll come back to that)... we can help you gain an understanding about exactly what you can expect to accomplish with the gear you currently have. (You will then be able to know when it is time to spend more money). We will approach this from the standpoint of NOT BUYING anything additional just yet. Let's see if you can work with what you already have... this way you'll know exactly what to upgrade and if you actually need to upgrade your gear.

We'll start with the premise that you can, with what you've already got, do what you need. When you spend more money, naturally, your options and the elegance of the operations do improve (but you would expect that to be the case).

You are working with an entry level keyboard and an entry level audio interface, but you will still be able to get stellar results. Results that should blow the minds of not only yourself, but all your friends and family. In other words, there is no need to apologize about the "entry level"-ness of the gear. These are the entry level to "pro gear"...

The MX is based on the Motif sound set, so it is no joke, and your audio interface has top shelf components - just because it is only a 2-in/2-out audio interface, it is still top quality! No apologies necessary! The quality will be awesome.

The MX has its own audio interface built-in, this means it can deliver audio directly to your computer via USB (without the need for using channels on an external Audio Interface, like your UR12).
Let's start with the MX as audio interface - the signal that travels to the computer via the USB connection is digital signal (zeros and ones). You cannot hear digital signal. So the signal delivered to the computer is digital audio. The MX is a 2-in/2-out audio interface. What that means is the computer 'sees' it as two channels coming IN and can send two channels to it going back OUT. When the MX connects to your monitor speakers - it does so as two different devices... first, as your synthesizer, and second as your audio interface.

The UR12 is an audio interface. it is a 2-in/2-out audio interface. It does not have a dual role, it simply is an audio interface. The computer sees it as two channels coming in and will be able to send two channels going back to it.

The MX is both a synthesizer and a 2-in/2-out audio interface. The two channels are the left/right out of the synth engine. The two coming back OUT of the computer go to the speaker outs.
Your UR12 is a 2-in/2-out audio interface.The two channels are the mic and hi-z input on the UR12 front panel, the two coming back OUT go to the speaker connectors on the back panel.

This begs the questions:
What is an audio interface?
Why do you need more than one audio interface?
If I only need one audio interface and I have two audio interfaces, how can I use both? What do I do?
Can I connect both audio interface devices and use them without a big effort?

Okay, let's begin:
The AUDIO INTERFACE is a device which takes in analog signal from your sources (keyboards, Microphones, guitars, etc.,) and converts analog-to-digital signal and OUTPUTS that signal to the computer. It also takes in digital signal from the computer and converts digital-to-analog signal and then OUTPUTS that signal to your monitor speakers.

The MX is an audio interface for itself.
It has the unusual property of being BOTH a sound source AND being an audio interface for itself. It has no other inputs. So when you connect the MX to your computer and you *select* it as your primary AUDIO INTERFACE, the computer 'sees' it as 2-inputs. (A Left and a Right audio channel which is the OUTPUT of the synthesizer engine!). By selecting the MX as your primary AUDIO INTERFACE, when the computer routes signal OUT to be heard, you send a STEREO (2-outputs) digital signal back via the USB to the MX where it does the digital-to-analog conversion and it sends that signal ultimately to the monitor speakers connected to the Main Left and Right outputs of the MX. The audio coming OUT of the computer can be MX recorded sound and all other audio in the computer (including your vocals, sound effects, etc. all audio).

The UR12 is an audio interface for other devices.
It has no synthesizer engine, it does not make any sound of its own, but it does take one or two analog source inputs and sends them digitally to your computer via USB. When you *select* the UR12 as your primary AUDIO INTERFACE, the computer 'sees' it as 2-inputs (probably 1 and 2). By selecting the UR12 as your primary Audio Interface, when the computer routes signal OUT to be heard, you send a STEREO (2-outputs) digital signal back via USB to the UR12 which does the digital-to-analog conversion, and then sends the signal directly to your monitor speakers connected to the UR12's Left and Right outputs.

What you MUST have:
The primary (*selected*) audio interface must also be the device connected to the MONITOR SPEAKERS - because as we stated early on, you cannot hear digital signals (zeros and ones) they must be converted digital-to-analog before we can hear them. So the primary AUDIO INTERFACE must be connected to your monitor speakers.

This means, all other devices (all your sources) ideally, would connect to the one primary AUDIO INTERFACE. This way you can hear all sources and you can control what you are doing while you are recording. You may also want to hear other items that are connected, as when overdubbing - so everything must connect to the primary audio interface (if you wish to monitor it, as in: hear it).

Now, here is where a larger audio interface would be more elegant. Doesn't mean you cannot work with what you've got, but this is the point where if your primary audio interface was able to handle more simultaneous inputs you would not have to render audio and/or swap/switch cables going to the speakers.

What it depends on:
YOU. Of course. What do you need to record simultaneously? If you play and sing, you may want to have both your microphone and your keyboard set to send data to the computer (Cubase) at the same time. This would require that you hear both your playing and you hear yourself singing. So both signals would need to be feed to the primary audio device's audio outputs.

Your current equipment does not allow for this contingency. A UR44 would allow you to connect the audio L/R outputs of the MX to two channels while the microphone could be connected into a third input. You could record the MX and your vocals simultaneously, you could hear both the MX and yourself singing by being able to route signal in the primary Audio interface (it has all signals and is sending those to the computer to be recorded, and is routing the combined signal to the analog monitor outputs, to be heard, as well.

What if you don't need to sing and play simultaneously. Then you do not need to record every thing at once. Then you can *swap* principal Audio interfaces for each task. And this is a viable way to work, when you don't have the budget to get MORE (elegant) gear. How this works is, you typically setup with the MX as your primary audio interface. You lay down your MX (music backing) tracks... I see that you record MIDI, first.... fine. Record your MIDI Tracks get them exactly as you like... then create a temporary AUDIO mixdown File (this can be done using the FREEZE function you may have read about) or you can simply render a quick stereo Audio File that you will use while overdubbing your vocals.

The only purpose of this file is so that you can hear the music tracks as audio, (this allows you to MUTE the MIDI tracks) - now you no longer need to hear the audio outputs of the MX live... the audio file will take their place while using your UR12 as the principal audio interface to OVERDUB your vocals. Otherwise you will not have a signal path to playback the MX... our UR12 is taking in the microphone signal. You have no path between the analog outputs of the MX and the monitor speakers! That's why the temporary audio file! (it is similar to how you work with VSTi - you make a temporary audio file so you reuse the engine).

By creating a temporary audio file - you do not have to worry about how you will hear your MX while you're connected to the UR12 as your primary audio interface.
The (temporary) audio file you made of your MX MIDI tracks will playback just fine through the UR12, and will allow you to monitor your music track while you overdub your vocals or guitar, or whatever else you use the UR12 for.

once you have all your Tracks assembled you can finish your project.

While the MX is your audio interface, you will be able to playback all recorded audio on the computer through it.

ALTERNATE SOLUTION:
If you have a Macintosh computer, you may have heard, you can activate multiple audio devices, by building an AGGREGATE AUDIO DEVICE, one that in your case would build 4-inputs/2-outputs. You would be able to receive audio simultaneously from both the MX (2-ins) and the UR12 (2-ins). You would still have the issue of monitoring - but you would be able to configure it so that you could monitor through Cubase (you set up a track that allows you to monitor post traversing Cubase). But to monitor yourself *direct* you would still would need to connect the audio outputs of the MX to the audio inputs of the active audio interface in order to hear it under zero latency conditions.

So Overall CONCLUSION:
The primary Audio Interface is the one that you select in your DAW. In Cubase this would be the one that is the "VST AUDIO SYSTEM" Driver selection (ASIO Driver).
Notice that when you have two devices (UR12 and the MX) that use the same Driver, you still need to designate which ONE of the two is the primary device. It is the one that the STEREO OUT from the computer needs to go to, because the AUDIO INTERFACE feeds the speakers. so only ONE of the two devices is both IN and OUT. Both are INPUTS to the computer, but only ONE of them is the OUTPUT from the computer to the speakers.

So I am referring to the ONE that connects to the speakers as your primary audio device. And in order to monitor (hear) directly anything, it must connect directly (analog) to that audio interface device... You have a DIRECT MONITOR button on the UR12 - its purpose is to allow whatever is connected to the UR12 to go directly to the speaker OUTPUTS... whether or not you record it in the computer is a separate routing.

DIRECT MONITOR literally means that the inputs are routed directly to the output. (it does not pass Go, it does not go through the computer, it goes directly from analog input to the analog speaker outputs). This is consider the ZERO LATENCY or direct path. A separate, converted analog-to-digital signal path is generated for the USB connection to the computer.

So Question for you:
Are you working on a MAC?
If yes, we can help you setup an "Aggregate Audio Device" interface.
This means all four inputs will be available for simultaneous recording.
This also means you can configure things so that the UR12 remains your principal (primary) audio interface - and you should be able to monitor the MX through Cubase.

Let us know, we'll help you with this. Worst case scenario, you will need to swap the cables to your speakers back and forth, manually. Connect the speakers to the MX when doing music tracks, and to the UR12 when doing Audio overdubs. It is a convenience thing... this is why I use the term "elegant" - it is more elegant to have one configuration and not have to swap audio interfaces and cables - but it does cost a bit more - that is the definition of ELEGANCE. 🙂

Thanks for the in-depth and quick reply! 🙂

I could have stated a few more details about my current setup. I'm sadly not a Mac user, so the Aggregate solution is off the table. I'm currently not using any monitors, I make do with just one headset. I could probably make do as well with switching back and forth, though convenience and elegance are both attributes I highly desire.
I write well, eclectically. Some songs starts with the vocal, some with the guitar. Some songs I've got more or less the entire framework for, and others I need to evolve through laying vocals on top.

I'll check more into the "Freeze" function and think about it over the weekend. See how I can work with what I have.

Cheers!