I have an existing back to back, looking at one side, the drain comes over from the left where the vent stack is and has sort of a street bending t that stubs in for each sink. Someone said this is not great cause if one sink backs up it will come in to the other. What is the correct way to rough in these drains and is there anyway I can get a free diagram?

Back-to-Back sink drain installation is a common - and up to Code - way of rouging-in two sinks.

Yes, if drain is obstructed pass that connection, water will back up to both sinks. You could go out of your way and install 2 separate drains - one for each sink - but you still will have to combine them somewhere down the slope.

From my experience, I have never heard anybody being worry about back-to-back sink connection nor have we had any problems with such installations in the field. So as far as my advice goes - do not worry about it. Your friend is over-thinking it.

Milo

Hey milo thanks for the answer. What was throwing me off was the old galvi fitting shape(looked like a mcdonalds sign or something} newer fittings make it look more right. Thanks again bill'