I think Sergiy was just giving a word of caution, to make sure to include the join. I looked at the original code I posted, and I also have an equajoin. And no, there can't be more than one poid in inserted or deleted (in my case).
I also want to make something else clear, incase someone else was confused like I was. That msdn article I quoted earlier says this, "[The inserted and deleted] tables are used to store the before and after state of the table...". That is kind of ambiguous, because I thought it meant deleted contains the entire table before any updates were made. Well I did some testing, and deleted/inserted only contain the result set of affected rows from your update statement. If you update 1 row, there's only one row. If your update affects 10 rows, deleted has the previous state of those 10 rows, and nothing else.