Hi
I am no expert. But mame is emulating the actual arcade machine. So in arcade games such as galaga there was only one joystick so not sure there would be a way for it to emulate a 2nd joystick
There was as you noticed 2 start buttons though.
I have not tried this but what u could do is go into the mame configure screen. Hit tab with a keyboard plugged in. tell it to use a button on your 1st controller for 2 player start.