If the pipes are sized and vented properly this should not happen even
though it can. In your case, the open P trap is lower than the basin so it
is certainly more likly than otherwise. In fact, the bottom of the P trap
is lower than the connection to the drain, the flow may be inevitable,
there's no uphill, just a short 2" hop across the pipe.

You may want to just run a snake down the drain from both sides just to make
sure there is no partial obstruction slowing the flow and waiting to clog.
Good idea to do this once a year or so with bathroom drains. hair and
personal products clog drains fast and bath sink drains are often only 1.5"



"Ray K" wrote in message
...
I have two back-to-back bathrooms. Their lavatories are also back-to-back,
and the same distance from the side wall. If I run the faucet in bathroom A
while the P-trap in bathroom B is open (waiting for a repair part), the
water from sink A will run out of the open trap in bathroom B. Is this
normal?