[vikingerik]

It depends on the emulator, across pretty much every platform that uses a joystick or d-pad, but in my experience the default most commonly is no left+right input at the same time. (It's semantics as to whether the implementation is "allow left+right input" defaulted to off, or "prevent left+right input" defaulted to on.)

[mikepurvis]

I'd also be interested to know what ends up happening if there are simultaneous presses— I assume last-one-wins would be the least surprising strategy for resolving it, but certainly several options are possible, and what is preferred might even be game-dependent.