can you use one of those snes boards with two mask roms to use two 27c040 to create a 1MB repro? I know people would usually use them to make a 2MB with two 27c801 but I have none of those and many 040
if you can, how would I split the rom?
Not easily, you'd have to modify the decoder so that it enables ROM2 past >512kB into the ROM instead of 1MB (so at A19 instead of A20). That alone would be tricker than just getting a 27c080 or a eeprom adapter board.
Rewiring a small DIP IC is no harder than rewiring for a ROM.
If a board with a MAD-n instead of a 74'139 or 74'00 .... you'll need
this.