I try renaming the vmu to something like "vmu_save_B1.bin" but that didn't do anything
Doesn't work currently, but support is on the way (see #1288).
Ill add the Android side when you give the go ahead @baka0815
I hope to get the refactoring done today, but that's no promise. Need to test on Windows first.
So this works now? How do you enable it on Android?
It's only possible on Linux currently.
Not for long ;) https://github.com/reicast/reicast-emulator/pull/1312
To answer the original question, open "Input" and select them from the dropdown list. It does, however, currently require a controller. This is not unlike the actual hardware, though.
Figured it is better to report here than open other issue:
The additional VMUs are not working on latest beta even with a second gamepad connected, also tried renaming one to "vmu_save_B1.bin" but it still doesn't detect the vmus even when the second controller is detected:

Fixed by df2d4fa
Great works devs.
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Ill add the Android side when you give the go ahead @baka0815