with latest svn, the following is now possible in MESS:

given that it's not possible for carts to enable subslots, I had to create a new driver "snesst" which (similarly to snessfx) emulates a SNES+Sufami Base Cart and offers 2 cartslot to the user to mount the st carts

More screens

some games work, some freeze at start.

also, mounting two carts is acknowledged by the emulated systems, e.g. because it warns you about carts inserted in the wrong slot

and it sees both mounted carts

but I don't think all data are read correctly...

I also had some work on the BS-X memory mapping which seemed promising (the CPU was able to reach reasonable code), but it turned out to be busted. so I threw it away and I left only the BS-X cartslot work. Due to device limitations, also the bsx unit is emulated with a separate driver: snesbsx offers two cartslots, the first for the base BS-X cart or for a compatible cart (like Same Game) and the second for the flash cart. However, the driver does nothing at the moment.