The historical standard configuration is to have only one audio interface at a time. If you're transferring audio from MODX over the USB cable - then MODX is your audio interface. Not all is lost, however, for Mac OS (as opposed to Windows). Apple's Mac OS supports aggregate devices (combining multiple audio interfaces). https://support.apple.com/en-us/HT202000.

I don't have an Apple computer (using Mac OS) setup - so I can't run you through this. But more OS-side setup may be necessary in order to realize what you want.

Optionally, you could use the Apogee as your only audio interface and run the analog outputs of MODX into the Apogee's inputs (using the breakout cable). This reduces the amount of flexibility in isolating PARTs between USB channels - and also reduces flexibility in automatically stripping the master/system effects as the USB outputs (from MODX) do vs. the Main L&R analog output. But this may also work for you.

I have read everything I can find online about MODX and MONTAGE CONNECT. Some seem to be missing important pieces of information. As I understand it, the MODX sends MIDI data to Logic, then routes it back to the MODX for tone generation, but how it gets back to logic, and out to my audio interface is unclear to me. And I don't want to use the MODX for my audio interface, i want to use the Apogee. If anyone recognizes where to go from here, I'd love to read your thoughts.

You have the signal flow correct in that the MODX will send MIDI data to Logic. You can opt to record the MIDI data to Logic — you would do so in a scenario where you wish to manipulate or edit the data in some fashion.

Recording MIDI allows you a degree of changing your mind about instrumentation, it allows you a way to correct your performance as you build up multiple tracks. In a MIDI Record workflow you would setup to record to Logic MIDI Tracks. The output of those MIDI Tracks will trigger the Tone Generator of the MODX which will output audio, you would connect the Main L&R Output of the MODX to a stereo input on your Audio Interface.

Your Audio Interface will connect to your Monitor Speakers.

In Logic you would need to set the Apogee as your Audio Interface. Wherever Apogee audio arrives into Logic that is where you would hear your MODX audio. When recording MIDI data you would work with Local Control = Off in the MODX. The MIDI Track in Logic would be responsible for receiving the MIDI from the MODX on a specific Channel and echoing the MIDI Out on that same channel to the MODX.

Unless this routing circuit is complete there will be no sound. Follow the instructions of your audio interface for monitoring options. The Analog audio Outs of the MODX are connected to the Inputs of the Apogee, so getting that to your speakers will be an Apogee Setting.

We should mention that, another workflow would be to use Logic like a Multi-Track Audio Recorder. Where you simply record Audio Tracks to build up your composition. If you do a lot of MIDI Editing this may not appeal to you... but it is viable workflow.

Jason, thanks for the information about MAC OS's ability to use multiple audio interfaces. I will fully investigate that this evening.

Bad Mister, I will try the cabling/setting design that you outlined above and see where it gets me. You also said:

We should mention that, another workflow would be to use Logic like a Multi-Track Audio Recorder. Where you simply record Audio Tracks to build up your composition. If you do a lot of MIDI Editing this may not appeal to you... but it is viable workflow.

You're correct in that I really enjoy the editing power associated with MIDI. However, at one point over the last few days I tried to simply connect the MODX8 in "Standalone Mode" to the Apogee to attempt a basic Audio track recording. This technically worked, but I got a lot of buzzing in my monitors which fully translated to the audio track in Logic, wrecking the recording. In fact, I could actually hear the superknob pulsing, as though my monitors were picking up the sound of the MODX8's internal electricity, even with the monitors at low levels (not sure if this makes sense). I'm not sure if there's a setting in the MODX or Duet that I would need to change to avoid this. My concern is that I will get this same distortion when I connect the analog outs to my audio interface when I attempt to recreate the design you described above, but I will know for sure tonight. Really appreciate the feedback so far.

You're correct in that I really enjoy the editing power associated with MIDI. However, at one point over the last few days I tried to simply connect the MODX8 in "Standalone Mode" to the Apogee to attempt a basic Audio track recording. This technically worked, but I got a lot of buzzing in my monitors which fully translated to the audio track in Logic, wrecking the recording. In fact, I could actually hear the superknob pulsing, as though my monitors were picking up the sound of the MODX8's internal electricity, even with the monitors at low levels (not sure if this makes sense). I'm not sure if there's a setting in the MODX or Duet that I would need to change to avoid this. My concern is that I will get this same distortion when I connect the analog outs to my audio interface when I attempt to recreate the design you described above, but I will know for sure tonight. Really appreciate the feedback so far.

Well, I can tell you that is not right at all. Buzzing is not a part of anything in this system. It sounds like you have a major ground issue. Can you describe the buzz?

The system buzzing registers on the recorded track, is that right? you can see the Waveform of the buzzing? So when playing back, it stops when stop playback, is that right? Or is it buzzing even when stopped? Is it a low pitched or high pitched buzz? Does it go away when you disconnect the Apogee from the computer? Does it disappear when you disconnect the MODX from the Apogee?

I had the same problem as you, i use Logic too.
When you record only audio, and want to use the Apogee interface, you don't need the USB cable from the MODX to be plugged in. If you want to use the MODX as the audio interface, connect the USB cable, then connect the MAIN OUT's directly to the speakers or use headphones plugged into the MODX.
As for MIDI, it isn't mandatory to use MODX CONNECT(although it is easier to recall performances), but to not have the buzz, you have to use the MODX as the audio interface and, as i wrote above for audio, to connect your main out of the MODX directly to your speakers or use headphones connected to the MODX.
Here is a tutorial for Cubase (very similar to Logic) -
https://www.youtube.com/watch?v=ZPPJeU9Ayy4
Logic with MONTAGE CONNECT (same as MODX) -
https://www.yamahasynth.com/montage-category/connecting-montage-and-logic-pro-x
And an article which explains MIDI with the MODX -
https://www.yamahasynth.com/modx-category/mastering-modx-modx-midi-settings-explained
Let us know if you solved the issue.
Cheers