I would like to replace two 30 year old toilets in second floor, back to back bathrooms. I am told to watch out for siphoning water out of opposite toilet bowl when flushed. The drain setup is as follows; 3" line comes up from outside wall, makes a 90 into the ceiling bay, runs flat about 16-18" to a 3" cross tee lying horizontally. The cross tee goes to each toilet with pipes running flat until elbows up to toilet flanges. The "top" of the cross tee continues flat in the ceiling bay about another 6-8" to a 3x3x2 ty where the 3" vent then goes up vertically to outside and other vents tie in on the 2" connection. Anyone have experience with this type of setup causing problems with newer toilets??

I have no experience with that set-up as it is totally illegal.

Your description indicates that you have a 3" vent that is unwashed within the horizontal run of the vent before it turns vertical and that the same horizontal portion accepts waste. Chances are that at least a portion of the horizontal vent has already accumulated waste within it, affecting the fixtures downstream.

he also has a double san tee on the flat.