a friend of mine has two bathrooms upstairs back to back. when he flushes one the other starts to drain down. this works both ways. we snaked the vent pipe but the problem still persists. is there a way for, say like a plastic toy or small ball to hang up in the double sweep of the drain that would cause this? they both work fine independently but the one that is not used will lose a little water in the bowl each time the other is used. the sweep in in the kitchen ceiling and then turns about 1 or 2 feet to the wall and then down. there is not another air trap other than in the toilets is there? what can be done to fix this problem?