I remodeled a 50+ year old house. There have always been back to back toilets, working fine. I replaced toilet #1 with a new Toto low volume toilet. Both still worked fine. I replaced toilet #2 with an identical Toto low volume toilet. Both flush fine, but when #2 is flushed it pulls water from the bowl of toilet #1. If #2 is flushed again it pulls MORE water from #1. Eventually the "water spot" in #1 is so small it doesn't work right, but if I flush #1 then it fills to the proper level and we start over. When toilet #1 is flushed, the water in toilet #2 moves a little, but none is pulled down. Both sides of the cast iron "Y" under the house are identical. Why doe this happen? How can I fix it?
Thanks

This is a vent problem. The discharge from #2 sets up a vacume that's relieved by the vent, But in this case toilet #2 is attempting to vent through toilet#1. Check for a blocked vent. Good luck, Tom

Tom is 100% correct. Is there even a vent for the toilets?

Tom,
Thanks. How do I check the vent? Can I run a snake down from the roof? It is a 1 story house.

I don't know how the vent would get blocked. It is a single vent in the wall between the 2 toilets and is the same vent that has worked for 50+ years. Is the fact that the problem doesn't happen in reverse when #1 is flushed a clue to where the blockage might be? I did redo the rest of the vent system as part of the remodel, but I didn't touch the main stack or the "Y" between the toilets.
I appreciate your help.
Gary

Not knowing your system I can't tell you why it doesn't work in reverse but if your bathrooms are back to back , typically, lavatory vent #1 will combined with lavatory vent #2 to form a single vent through the roof.
Now if the branch vent from lavatory #2 were blocked BEFORE it joined with lavatory#1 that would set up the situation that you now have. That's the best I can come up with. This presents another problem. This can't be snaked from the roof vent. You would have to open up the wall and cut into #2 branch vent before it joins #1 to snake it.
That's my take on your problerm. You can run this by another plumber but That's the only explainatioin and solution I can come up with, Good luck, Tom