I'm looking to replace the circa 1990 toilets in the second-floor baths of my home; I think the existing units are 1.6 GPF, made by URC (?). The bathrooms are in a sort of back-to-back configuration with a horizontal "W" connection (see drawing). The W fitting runs parallel to the floor, with the discharge end taking a 90 downward to the drain, and the center opening taking a 90 upward to vent through the roof. The wet wall is non load-bearing, with the kitchen ceiling immediately below.

I've read on this forum that back-to-back toilets using a sanitary wye can interfere with each other, particularly with the newer models with 3-inch flush valves. Is this configuration going to be susceptible to the same sorts of problems? Should I stick with replacement units with 2-inch flush valves to minimize any siphon action on the "other" unit when one toilet is flushed? I've been considering Toto, would need an elongated bowl (compact would be nice but not necessary).

Re-working the drain connections will be problematic due to the amount of tear-out and re-work necessary. We've never seemed to have any interference issues in the 20 years we've lived in this house ...