Currently planning the layout of an upstairs bathroom that I'm adding. I've done plumbing before without problems, but I'm questioning the engineering on this one. The 2nd story bathroom I'm adding is directly above the 1st story bath. Currently, my plan is to drain the toilet down the stack, vent all fixtures to the stack, and run a seprate drain to the basement for the sink/tub. I have two plumber friends, one says that's fine, the other (who is more a pipefitter than plumber) says It might not work because it could suck the traps out on the 1st floor when the waste comes down the stack. I've never done a second story bath where I could use the stack this way. It's possible to run another 4" drain for just the toilet, but if the other stack will work that'd be silly. Just looking for additional opinions. Thanks!