I am quite curious about this as well (preparing for a psx build soon). A little digging and I found this snippit from the retroarch.cfg located in ~/RetroPie/configs/psx/
# Toggles eject for disks. Used for multiple-disk games.
# input_disk_eject_toggle =
# Cycles through disk images. Use after ejecting.
# Complete by toggling eject again.
# input_disk_next =
Did you uncomment these lines and add key values? It seems like you begin the process by “ejecting” the current game, cycle through the available images, and finish by using the eject toggle button again.
Unfortunately, I don’t have the psx set up yet so I can’t verify this works, but it seems pretty straight forward. Let us know if you get it working.