Plumbing - lavatory drains

If they are installed with a "cross" or "X" fitting in the wall, likely water will gush across the opening and come out the open trap on the other side of the wall.

yes

Back-to-back fixtures are plumbed into a Double Sanitary Tee, like this one:

Under low flow conditions, waste might not cross over, but a sudden draining of the basin might spill some out.

Jim

Yes, especially if there is a lot of water draining in sink A.

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"

Good call, Wayne. Thanks.

Jim

Flogging a dead horse, here's an interesting discussion of double Tee's and the application of the 2006 Code requirements:

That would certainly prevent the problem. The house was built in 1969, so maybe it wasn't required back then.

In response to others, the water flowed out the open trap even with modest amounts of water running in the other sink from simply turning on one faucet. In other words, I didn't fill one sink then try to empty it quickly by raising the popup suddenly.

Now that the p-trap is reconnected, all is working properly.

Thanks to all who responded.

Ray