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Without touching core files, just be making a module, it should be possible to have it register on the other software, and logon. Of course this will require the 2 DBs and all the software is still there, just that bridge, like you said.
I think the module would need the brunt of the coding so as not to affect any core files on either software, although I know the main register and logon functions on the second software would need some modification.
Well, just a thought anyway, I think it might still be a plausible way to go for some integration until all modules are effectively available within xoops.