of course you can double knit in the round… (in some ways, its easier–you keep track of pattern on front, (never turn!) and the inside takes care of its self!
you can use any fair isle pattern–
decreaseing is bit tricky (for say the crown of hat)
you need to use a cable needle (and position 2 knits together (to K2tog) and 2 purls together (to P2tog)
more complex decreases (double decreases ) are possible… but… well, more complex!
if you are on ravelry, there is a double knitting group–and many members of the group have “object patterns” (my blog has an 8 part tutorial (link above is to one page!) and i have many stitch patterns… (vs object patterns) my stitch patterns are over sized swatches and/or potholders…
[U][B]The Big Book of Knitting[/B][/U] has a some good info on double knitting too…