I am working on plumbing in my garage and trying to find out about a Sani flo toilet on my 2 inch drain in my garage that connects to my house plumbing. The toilet and sink drain both have a t coming out of the wall one on top of the other and I was thinking the toilet needs to be the bottom one. But I also am told I need to put a in the wall vent in as well. So I have a pipe ran up about 8ft so far. I am wondering since the toilet pumps out of 3/4 I'm assuming it's pressurized. Would the sink and vent cause issues with the Saniflo trying to pump since its vented once it hits the 2inch pipe for 15 ft before it hits the 4 inch. The 2 inch is sloped correctly for the sink drain and the toilet is an after thought

I think you would be better off in the long run to increase the 15' run of 2" to a 3 or 4 inch line and install a normal toilet.

My code does not permit a kitchen sink to share the same branch with a toilet.

I don't have much of a option on increasing the line size. If I could I would like to. The garage is detached 6 ft away from the house and is under concrete 4 ft down all the way to the house. The sink is a double utility garage sink