Sorry, I'm possibly being dense but I'm not sure how dividing one file into two helps you with your lack of storage, especially since OBB files contain game assets (accounting for 99% of the file size) and that would need to be updated every new release anyway as I include new assets in each release.
I think what you're really asking for is a patch file, but unfortunately neither Ren'py nor Itch support Android patching. It's a ridiculously complex procedure that people in the indie dev community have been chipping away at for years with no success :(