Exo probably hasn't implemented multiplayer support in gpSP as it would be more difficult than multiplayer in which there is only a single console involved. I know that TGB-Dual goes about this problem by emulating both systems on a host PC and from there working the same as a typical emulator would with multiplayer, by sending output from the first player to the second and keystrokes back from the second to the first. Evidently, the PSP struggles emulating only one GBA, so a solution like the one I mentioned in unfeasible.
So, in a nutshell: It's just plain harder to do multiplayer support for a GBA emulator than "every other emu" that has said support.
Forgive me if I'm not 100% correct on the details, I'm evidently not a coder myself, but at least people may notice there is indeed a reason Exo doesn't feel like implementing multiplayer due to this post.
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