What if the trigger was a loop ? The trigger could release the sear either by moving forward or backwards......The disconnector would hold the hammer until the gun is closed into battery and as you release the rearward preassure to reset the trigger a subsequent forward push would immeditately fire the second round......thus the need for the loopy trigger.
If I'm understanding you correctly (pull the trigger fires one round, release the trigger fires another) I think BATF has issued an advisory opinion that those are legal. I'll have to look around and see if I can find a copy of that letter online - I do remember reading something to that effect. Come to think of it, I have a friend who has exactly that type of device on his Mini-14.