Two toilets plug on the same line?
Sounds like the line.
I would pull the toilet up and get someone in with a large snake for the waste line, the kind for main lines.
You can then test the toilet on blocks outside and make sure it's not the trapway of the bowl just to be sure.
When we set toilets, we always set the wax on the flange, and then drop the toilet onto the wax.
A wax ring can fall off and drop over the drain opening otherwise.