Just finished remodeling both first floor bathrooms with TOTO Drake elongated ADA toilets installed. Toilets are installed back to back using existing double sanitary tee. When toilet is flushed, water in the bowl of the other toilet is partially siphoned out and preceded by a large air bubble. Bowl can be refilled by an additional flush. My plumber is dumbfounded and has no fix. Vent pipe is clear. Septic tank recently cleaned. Never had this happen before with my old 1972 toilets.
I spoke to TOTO tech service rep who recommended replacing the double tee with a double wye fitting for this G-Max toilet. Will this solve the problem?
Need some advice on how to fix this unacceptable installation.