I had this combo for a while – honestly it was pretty clunky. For one, you need some kind of mixer to blend the voice from the Mopho and the voices from the Tetra, since neither has an external audio input. As I recall (and forgive me – it’s been kind of a long time) the control from the Mopho via polychain doesn’t extend beyond keyboard control and cycling the voices – in other words, you don’t automatically get a matching patch on the Tetra if you select a patch or make changes on the Mopho. If you want the same patch, you have to manually match them on both units – I believe they share some presets, but otherwise it would mean reprogramming the Tetra to match what’s on the Mopho, or vice versa. It is possible to use a “Dump” command to transfer a patch from the Mopho to the Tetra via MIDI, but again this is a manual operation. So you could theoretically program a patch on the Mopho as a mono patch, transfer it to the Tetra to see how it sounds as a poly patch, and continue editing and transferring until you get it right. One thing that might make all of it easier is getting the software editors for both the Mopho and Tetra from Soundtower.
I ultimately sold both my Mopho and Tetra, but ended up buying a Mopho x4 later, which is pretty fun. It’s not as easy to program as more recent synths but it sounds great, and it’s way easier to get polyphonic sounds even if it’s one voice short of the Mopho/Tetra combo. Unless you’re getting a shockingly good deal on the two together, I’d be inclined to look at something like the Take 5, which gives you the same voice count but an obviously much more organized interface.