Multiple pipes can lead to the same location, though. You can overlap pipes.
So you place one pipe and put Mario through it, and place the exit, then go back to the main area and place another pipe and put Mario through it, then place the new exit on top of the old one.
Both main area pipes will make you come out the same sub area pipe. Going back into the sub area pipe will make you come back out of the first one you placed.
Cool! Thanks for that. Pipes can be devious then haha.