Both ROM chips are connected only to the S-DD1, and the S-DD1 is then connected to the SNES bus. That's how it does its real-time decompression.
http://www.snescentral.com/pcbboards.ph ... VC-LN3B-01You map in the upper 2MB by using the built-in MMC, which acts a lot like NES memory mappers.
It was of course totally unnecessary (and in fact only made it more complicated to program the game), given the SNES address bus can easily accommodate an ~11-12MB ROM without any MMC. But what are you gonna do? FEoEZ did the same nonsense for a 5MB ROM.