I'm replacing an old cast iron p-trap with PVC and while I'm doing it, I thought about installing a Y joint to run another piece of 2" drain up to my top floor where I"ll be building another shower. I figured since I was already replacing the trap and tying it in to the cast iron drain, it would be a good time to go ahead and put in the Y joint since it'll have to be done anyway to tie in the new shower on the second floor.
My question is, should I put in the Y joint then cement a run of 2" drain and cap it off until I'm ready to start the upstairs shower (in about a month) or should I finish off the drain from the shower I'm working on and then in a month just cut it off and put in the Y joint then? I'm trying to save from having to go back over what I already worked on. I guess I'm concerned that it would either inhibit good drainage or maybe trap sewer gas. I'll try to take a photo to illustrate better.
There's no reason why you can't do that. Just make sure that the wye portion of the fitting has grade on it (check with a torpedo level for about 1/4 bubble)
Make sure you have enough pipe to glue to later.
The only other thing to do is make sure you're not cutting your wye inbetween the trap and the vent. That would constitute two traps on one trap arm, otherwise known as "Double trapping", which is illegal.