Please correct me if I'm wrong, isn't it better to stick to a single-target rotation if there are only 2 mobs?

Another time you can use flare is at the end of your single-target rotation if you have swiftcast & convert up. Of course you only want to do this if you don't need the swiftcast & convert up later for certain DPS checks.

I wouldn't say never use Fire 2, because some tanks simply can't handle Flare spam, unfortunately. You may have to use Fire 2's in situations where the tank can't handle you generating hate as quickly as you will using Flare spam.