If the question is “Can you...”, yes, you can connect the unbalanced outputs of your keyboard directly to the balanced inputs on your monitor.
If the next question is “Are there any drawbacks...” well, yes, a balanced connection into the speakers would be ideal. Using a small format mixer is often done in home studios.

The job of the mixer is not only to allow you to plug in more than one device, but it also can take in unbalanced inputs and output balanced signal, thus being a more ideal solution.

Unbalanced cables are as professional as balanced cables. The difference is when you need to run long distances, balanced cables will cancel out any radio frequency interference. The longer you make a copper wire the better antenna it becomes. Because the balance cable has three wires (a hot, cold and ground) the noise enters equally in both the hot and cold and gets cancelled at the end.

The longer an unbalanced cable the better an antenna it will be. When does this become a problem? Over 25 feet.— this is why you rarely see any unbalanced cables longer than 20 feet long.

For a Home Studio short cable runs are recommended and unbalanced connections direct to speaker is done... it’s fine... but realize that a mixer in between is better... not only because it properly prepares the signal but it is built to handle the types of inputs that would be coming from a keyboard. The all important INPUT stage of the mixer will possibly have an input indicator, a peak warning light, perhaps even a Pad. These are all designed to help you protect your gear in the next stage of signal flow.

Plug straight into your speakers with the full knowledge that a mistake could be catastrophic.

Speakers are forced to respond to whatever you send in... the mixer can act as a safety net. I find I have to warn guitar players that there is a proper order in which to connect and power on and power off your gear. So many guitar players crank their sound system and then plug in their instrument — as if that horrendous “boom” as the speakers reproduce 2Hz at unbelievable volumes is COOL (on any level, it is NOT) and would be particularly harmful to delicate studio monitors.

The last thing powered on and turned up should be the speakers. You never want to *hear* something being plugged in. And when powering down, speakers off first... because you never want to *hear* something powering down.

If you treat your ears (and your speakers) correctly, they will last you a long time — and continue to sound good.

If money is not an object, get yourself a small format mixer - it will serve to ‘prepare’ your various signals for your speakers.
You can connect without one, just be careful, you are plugged directly to your monitors (not ideal, but can be done).

Your MODX synth can be set to -6dB, +0dB, +6dB, or +12dB Output. Your Monitor Speakers can probably handle signals as weak as -10dB (consumer electronics) through +4dB... most will have a scale on the Input Sensitivity knob. Set it to +4dB (Line Level) with your synth set to +0dB this should be fine, if you need more volume, set your Input Sensitivity slightly more toward the -10dB

Recommended: a small format mixer that has inputs designed for keyboards, microphones, Hi-Z Ins like guitar or bass, may even have some RCA (unbalanced connectivity)... this will allow you home studio greater flexibility and allow you to handle more types of signals.

... adding to the mixer suggestion:

Some audio interfaces (boxes with balanced and/or unbalanced inputs, balanced/unbalanced outputs, sometimes MIDI I/O, sometimes headphone reference, sometimes integrated effects, always a high-speed connection with a computer such as USB today and firewire or other standards before) blur the lines between a pure "soundcard interface" and a mixer that happens to connect to a computer.

Some of these audio interfaces have "all" of the features of a mixer and more. Built-in insertion effects (reverb, compression, etc), EQ on all audio I/O, an array of balanced XLR/TRS/TS combo connections, hands-on knobs for adjusting per-channel gain and mains levels, etc. Typically the differentiator is how many knobs and dials you have access to on the audio interface itself - although some "virtualize" these with an app that can run on a tablet device.

Here, I have a mid-grade audio interface that acts as the "hub" of my audio chain. I still have my Montage connected to the computer through USB - but the audio path first goes to my external audio interface which gives me real-time knobs for each XLR/TRS combo input's gain, monitor level, headphone level, button-press per channel for various other options. Also each channel has a clip indicator which is the dummy light for input levels exceeding max levels. It has 4 XLR combo inputs, 4 unbalanced TS inputs, the ability to switch on phantom power (globally for all XLR inputs), etc. It's missing a few features of a full-fledged mixer but does not shave many off of what a smaller form-factor mixer would tend to include. This audio interface also lets me balance the levels between my computer (USB computer-to-audio interface level) and directly connected devices (Montage and another keyboard). I have never used the audio interface's built-in MIDI interface although that is there.

Having an audio interface may be something to consider that can cover similar bases as a mixer.

So I should be looking for a mixer that has 2 unbalanced TS inputs and 2 balanced TRS or XLR outputs...?

You should look for a mixer that is compatible with all of the devices you intend to mix - subject to finances.

And perhaps decide if you want any more channels for the future - or not - and take a stab at what those may be.