I checked other posts and I couldn't find one about this so I've decided to post this. If you're a druid you can cast Faerie Fire and Faerie Fire (Feral) on the same target without them canceling each other out. This can be useful to take down enemies faster in raids, PvP, or PvE. If this is a repost please don't flame me. I've been a leecher for a while and finally decided to contribute something.
*EDIT*
You must be specced far enough in Feral to gain the talent Faerie Fire (Feral)