Flash only one file: pg-multi.uf2 only one time. It contains all firmwares (6 at this moment). You can change between them in DOS:
New usage for this version:
pgusinit /m x [d] - change card mode to x (1=gus,2=sb,3=mpu,4=tandy,5=cms,6=joy)
the optional parameter 'd' makes the mode permanent at boot
(Only if pg-multi.uf2 is flashed)
Examples of use:
You have default mode as GUS mode on boot, want to change mode to SB, but only temporarily:
pgusinit /m 2
You want to change the default (permanent) mode of the card to SB mode, and the next system cold boots / hard resets / power off you want to stay in SB mode without using pgusinit:
pgusinit /m 2 d
Thanks to @polpo for the main code and @jeroentaverne for the great code of pico multiboot here!!: https://github.com/jeroentaverne/pico_multi_firmware