Essentially this is the thing. I have a return register upstairs on the 1st floor and one on the second. The one on the 1st floor has great suction. So much that I can put a plastic storage box lid on the wall and it will hold it there. However the one on the 2nd floor has almost no suction at all. It has just enough to hold a paper towel on it (barely). This is an old house using a wall-cavity return method. The wall cavity between the 1st and 2nd floor is shared. So I was thinking if I could put a vent on the 1st floor that limits the amount of air coming in it would pull more air from the 2nd floor. Is this ok to do. If not what are some better methods?