I have renovated most of our home myself with minimal difficulty but I have seem to run into a wall here. I want to make a new 1/2 bath (toilet and sink) which will serve the master bedroom. The space for the 1/2 bath is directly behind an existing bathroom. I want to install a toilet in line with the existing toilet. There is a 3" vent pipe running up through the roof. This vent will be between the two toilets. The toilet flanges will be approx. 30" apart (center to center). Both toilets will be on the same drain pipe which is under the concrete floor. The new toilet will be between the old toilet and the septic tank. I have cut the floor and the pipe is exposed and waiting for me to cut into it and install the new pieces. This is where I become lost. I am getting conflicting advice from many sources and I'm not sure which is the proper method to use. I almost installed a 3" saddle tee today but just don't feel right about it because the direction of flow is a direct 90 degrees into the drain line. When the toilet is flushed shouldn't the sanitary tee flow toward the septic tank? Will one toilet cause problems for the other being installed so close and on the same 3" drain? I'm also not sure at what point to install the flange. Should it be on top of the tile floor or on top of the concrete. If it's to be installed on the concrete should the toilet be installed on top of the tile or should I install the toilet on the concrete and then tile around it? I know, I know, what a rookie, but I'm determined to do the job correctly the first time. Any advice you can give me will be greatly appreciated.
Let me recap the install and direction of flow: Existing toilet --> 3" drain pipe under concrete --> 3" vent pipe in wall between toilets and through roof --> new toilet directly over 3" drain pipe --> septic tank. Also keep in mind that the two toilets are only about 30" apart.