I'm actually with Lacero on this one. I find it natural to use two posts to answer two (somewhat) different questions. It makes it a bit easier to follow a thread that way.
If the questions are similar (or otherwise connected, e.g. totally contradicting, which also is funny), and a (more or less) suitable response applies to both, of course I quote both in the same post...
I find the best way to do this is to hold down cmd while pressing quote on the two (or more) posts, thus opening them in different tabs. Then it's just a matter of cutting and pasting between tabs to get all the quotes into one of the replies and answer there.
I also tend to "double post" in very long threads if I answer to two different posts (even with similar questions) if they appear with several pages apart and (apparently) has no other connection... even if I then, ironically, make a long thread even longer than it, strictly speaking, had to be...