Hello,
to answer your questions:
- you can't chain two mopho desktops to the mopho KB. You need to select the number of chained voices on the KB, it only allows 1, 4, 5, 8, 12 or 16. So not 2 (unless they changed it in an OS update).
- chaining to tetra instead of mopho DT is preferred for two reasons. First, you'll have more voices. Second, you can't control the feedback of the mopho DT with the KB.
- if 4 voices are enough for you, you don't need the external input of the KB and you don't need the multitimbrality of the tetra, the mopho x4 is the better choice: more keys and no need for an external mixer.