This would be my question as well. If i go up, and pressing this button 2, then the output signal does not know if i'm going up or down or none of those.
Obviously it'd be better to have a protocol like the megadrive, but given the setup in the ST, this is a hack without having to change the ST hardware.
That works for Metal Slug for the grenade, because a) you don't hold down the grenade button, b) you don't have a lot of grenades so you don't use them that often so you won't really notice any weirdness like if you duck and grenade simultaneously the duck doesn't happen until you let go of the grenade button.
If the up+down button was used for firing the main weapon, this wouldn't work, especially when you've got the machine gun and you can just hold the button down. You wouldn't be able to duck or aim up while shooting, or it would be unreliable.
If you were trying to use it for a three button fighting game, I think you'd have problems too. Especially if you are doing 'negative edge' techniques where you would hold a button while inputting the directional sequence and release the button to trigger the special.
