What you are building is far closer to a gameboy than it is to an arcade game. No one was ever chomping at the bit to get a second gamepad to plug into their gameboy.
Once you build it with 2 player controls you have then sabotaged it by insisting on using a free tablet. That 800 mhz ARM processor is going to choke on anything past the classic era.
Classics like Bubble Bobble and Metal Slug are fine

I'll try what works on the tablet before building anything.
I'll go with 1 player controls only and add a port for external control if necessary. Some games, like the ones mentioned above, are best played with a friend
