Okay, I think I'm getting the hang of this now.
However: What do I do with the system sound signal? Now, the Ch1 and Ch2 faders on the X32 control
both the Windows system sound
and the first two faders in Reaper!
The reason for this is that system sound is returned on Card 01 and Card 02. As far as I can tell, there's no way to change that in the X-UF Control Panel.
Any suggestions?
EDIT: Think I found an okay compromise. A bit tired now, so I'm not 100% sure I'll not run into other problems, but what I did was swap Ch1 and Ch2, and Ch31 and Ch32 routing both on the X32 and in the routing matrix in Reaper.