Is it posible to play my drums with my right hand on the upper half of the keyboard and chords with my left hand on the lower half of the keyboard (as two separate renoise instruments, rather than just assigning the samples that way within one instrument)?
Yep, just open the MIDI panel of each instrument you want to assign to a split and choose your controller + the key range.
This should make the instruments pick up notes from the controllerwhen it matches that range, and otherwise pass them on to Renoise.
I’ve also created a little tool that allows you to do this in a more compact interface:
https://forum.renoise.com/t/new-tool-3-1-midiperformer/47001