The problem with using two needles at the crown is that you need to be able to work the circle; it's impossible if you only have the stitches on two needles AND you're trying to knit with one of them. You'll stretch the stiches like crazy. (I know because I've tried 2 dpns to prevent the needles' slipping.)

If you knit a lot of hats in the same weight yarn (or close), get some bamboo needles. They don't slip like the metal ones. (They also serve as fantastic cable needles!)