I was playing a game today with priest and sewers. After trashing a card with priest, I was able to followup and trash a card with sewers, but I didn't gain the additional 2 coins from priest for trashing another card.
This is not a bug, because the card instructions are resolved one after the other. So when you trash the card with Sewers, the +2$ of that Priest hasn't activated yet.
Here's the explanation by Donald X. himself:
http://forum.dominionstrategy.com/index.php?topic=19025.msg771931#msg771931