The no-BIOS path mapped SNES B to GB A and SNES Y to GB B, leaving the
SNES A button dead - so the same game handled differently in split view
and BIOS mode. The SGB2 BIOS's own translation (sub_80BCDE, via the
SGB2.sfc.lst disassembly) reads A to A, B to B, with Y doubling as B -
left of A on both pads - and X unused. Map the same way.