Multiple buttons with script is best done with Bitwise logic... or, if you really, really want to make it accurate to arcade games, you need to log the key, and then wait a few frames before you allow its default function to take place. During that wait time you monitor other keys that have simultaneous action with original key. If one fires you trigger the double press action instead.
KOF delays 4 frames before input while it checks for double key presses. I don't know how long Capcom games wait. By default OpenBOR doesn't wait at all, which is why simultaneous press detection can seem finicky at times.
DC