In another post I was planning on replacing a toilet based on a plumber's advice. The toilet won't flush (it tries, but nothing, or very little, goes down). He snaked it repeatedly, said it wasn't clogged, and said I needed a new toilet.

A friend with some plumbing experience suggested I first check the venting system for a birds nest or dead animal before ripping out the toilet. He said that the lack of proper venting could stop the toilet from flushing. However, there are two toilets in the house, and only one roof vent, so I assume they are linked. The basement toilet flushes perfectly fine. If one toilet flushes, but the other won't, is it safe to assume that venting is not the issue?