Pádraic maintains that one assertion per test is a rule that should always be followed unless there is a specific good reason to break it. I prefer it as a guideline, as does Robert C. Martin in the book Clean Code. The reference is not intended as an appeal to authority to "prove" that I'm right. I'm just making the point that I think this reflects the current state of the art, which is not necessarily perfect, of course.
Dagfinn talks about behavior-driven development and illustrates how a two assertions could be combined into one but that the end result "isn't very pretty" - and that's just testing two proterties of a single object. Imagine what would happen if things got more complex.