never mind... if fixed it! 1st i made a full .uae file like written in this thread. But that didn't work. Apparently the .uae only needs to have the drive settings and then it works. e.g. this for Canon Fodder i have this .uae file floppy0=/home/pi/RetroPie/roms/amiga/Canon Fodder/CANFOD-1.ADF floppy0type=0 floppy1=/home/pi/RetroPie/roms/amiga/Canon Fodder/CANFOD-2.ADF floppy1type=1 floppy2=/home/pi/RetroPie/roms/amiga/Canon Fodder/CANFOD-3.ADF floppy2type=1 floppy3=/home/pi/RetroPie/roms/amiga/Canon Fodder/CANDATA.ADF floppy3type=1 with nothing else in it, and then it works...

@supercatfooz Update: I just found an HDF file that seems to work fine, making the two .dim files I have obsolete. I'd still like to know if multi-disk mounting is possible for the future, though! (plus it'll help whoever else happens to have this problem)