Kale, MESSUI show me 2x Cartslots as devices. I think the second device should be called " Cassette ".
as long as it is coded as a cart slot, its name cannot be changed.
also, I start wondering if the .p6 and .cas are really representation of the content of the cassettes or if they only are memory snapshots of the loaded program (like .t64 files for the C64). if they latter, they are not cassette images at all and it would mean no real tape could be re-created starting from them...