fourth year
Our code says you can connect three toilets on a 3″ pipe as long as the third one is at least 15′ from the other two. But if I read you question properly, you are going to connect the new toilet to the closet bend of the existing one. If that is what you plan, then it is not allowed. Unless you also install a vent between the existing toilet’s closet bend floor connection and the new toilet’s combination Y-1/8 bend. And with that configuration, the new toilet would probably also need a vent from its horizontal pipe, since it would essentially be wet vented.
