Ah multi-timbrality! Check here:
This wish was originally for the OG Tracker and was declined as you can see.
But maybe now with the Mini / Tracker+ this is back on the menu?
If you want i could resurrect this wish and you can add your comment to it and/or rewrite/rephrase the original wish?
And now for the other wish - by the way for the future: please only do one feature per wish 
. Thank you! 
Anyway…
So technically polyphony on a single step. Multiple notes (even if it is the same note) played on the same step (but in your case just different instruments). Correct?
I’m not sure if this will be accepted, but i don’t think there has been a wish requesting this yet. So if you can, i’d say please rewrite this wish with this Layer functionality in mind.
As a workaround / idea for now - why not render/resample the two samples together?