thanks a lot for the answer, bootgod... however, I'm now a bit more confused than before
given that it seems very hard to find precise info on these dual carts, let me see if I have at least git the basic fact correct:
a. Nantettatte Baseball!! was a Sunsoft cart with a small slot where the user can insert a mini cart (I guess with updated teams, or something similar)
b. at least two mini carts were produced: '91 Kaimaku Hen and OB Allstar Hen
first questions: does the picture at your website represent the base cart? is the "sub-slot" visible in the pictures (I am not sure how it is supposed to look like)?
Concerning emulation, I'm a bit puzzled. I discovered that in GoodNES 3.14, two UNIF dumps of the minicarts were included:
Nantettatte!! Baseball '91 Kaimaku Hen (J) [U][!].unf
Nantettatte!! Baseball OB Allstar Hen (J) [U][!].unf
Analyzing these files, they both consist of the same PRG0 and CHR0 blocks (which are exactly the same as the PRG and CHR of the base cart) and of a unique PRG1 16k block
I guess these PRG1 chunks contain the additional minicart data (which would seem to contain PRG data rather than CHR... weird).
What somehow concerns me is that I'm not sure where this PRG chunk should be mapped exactly...
NEStopia source seems to handle PRG bankswitching in a very strange way for this board and it's not clear to me if it ever accesses banks beyond 0x10 (which would contain the PRG1 data, if it's loaded right after the base cart PRG). Or maybe the PRG1 data has to be loaded elsewhere and/or mirrored somehow.
I will probably spend some time to further debug these games in the weekend, but I hoped it could have been easier because there are several issues with higher priority in MESS...
A last question (even if I know that without a minicart to do experiments, it's hard to give answers): if the minicarts turn out to contain only PRG data, would you still expect 256k CHR? would it possibly be VRAM? does the PCB of the base cart offer any hint?