I am adding a toilet in addition to one that is roughed in so I will have two toilets in the basement when I am done. Do I tie in where the sewer comes in (adding a backflow valve)or can I tie into the existing 3" toilet rough in. Also, I have a middle foundation between the two toilets. Do I go under the foundation or have to cut through the foundation to run the toilet to the other side.

Are you just adding a toilet or will it be another bathroom group? I would'nt cut the middle foundation unless I was forced to. You haven't mentioned a vent for your toilet. What's your plans for venting it? Regards, Tom