Hello Blubaru and welcome to these forums. You'll find many helpful people here and a lot of useful information. Make sure that you:
I cannot tell from the Roland manuals how many audio streams are supported on the USB interface. Can you post a screenshot of the drop-down lists in Reaper's preferences/ Audio/ device?
How to describe your hardware set-up (in Post #3)
I suspect that it will show 2 channels in which cases the audio will be combined and cannot be separated. You could record one instrument onto one track, then Stop, select another track for recording, record a second instrument there (while listening to the first) and so on.
You should also set the Reaper tracks to the appropriate Input device (not the track Output).
On the other hand if you do see 11 input channels on the Reaper Input menu you could create 11 Reaper tracks, select a different channel for each and record all 11 in parallel. But, I'm guessing in the absence of facts.
