Multi channel audio interface, external synths and sidechain

Hi!
I want to use motherducker to compress an external synth. I guess the compression happens at the audio (not midi) stage so my current setup of a mixer going into a 2i2 focus rite is very limiting because I can't compress just the bass synth. Would a multi channel audio interface allow what I want? In my head I imagine I set the bass synth to output on channels 3+4 for example and set motherducker in on my kick drum and put the motherducker effect just on the audio channels I want compressed. Is this how it would have to work? All the tutorials I've seen assume that the target track for motherducker is an audio based one inside the MPC. Thanks.

example of a way to achieve what you ask:
mpc midi (or midi over usb) out ---> ext. synth midi in (or usb) ; ext. synth audio out---> mpc audio input.
go to audio tracks, on a free audio track set the input to the input you used (if mono choose 1 or 2, if stereo choose 1 & 2) ; set monitor of audio track to ON.
go to the channel mixer page and scroll to the left to find the audio track mixer page, select your audio track and insert mother ducker into one of the 4 slots.
now go back to main and switch from audio to midi (lower left corner of main page), scroll to your drum track, go to pad mixer page and locate your kick, put motherducker INPUT plugin on one of the 4 effect slots. remember you can have different motherducker\motherducker input couples, if you have more than one couple, put each couple to a different "bus" (inside both plugins).

Thanks for responding! Unfortunately this won't quite work because all my synths come in on the same audio channels. My current audio interface only has two channels and my synths are stereo so I run them all into a mixer first and then into the MPC (via my current audio interface) so that means that all my synths would get ducked, not just the synth I want to duck. However, from what you say, if I did indeed have multiple audio inputs on my audio interface then I could just choose the two input from the synth I want to duck?

You could still pull it off.. you just gotta record it in with the other channels muted on the mixer. Work 1 track at a time to record it all in. I actually prefer that method over simultaneously processing multiple tracks since my hardware processor stash is limited anyways. If you're recording the midi notes to be played by your synths and routing audio back into the mpc to capture it you're 90% of the way there.

Thanks for the suggested method!
I'm kinda set on my way of working though; I like to be able to have the whole thing performing live on the hardware and would rather throw money at the problem than change how I work lol. One of those Behringer UMC1820 devices looks like it would satisfy. I would be using 5 of the 8 channels currently and could add their ADAT unit to get another 8 if I ever needed them. Just need to look in the compatible audio interfaces thread on this very site to make sure it works well with MPC.