People definitely do it, but do not be suprised if your Crested is missing a tail very soon after. The main thing is, just keep an eye on them and make sure nobody is getting stressed and everybody is eating. Also, make sure they are always around the same size.
I'd love to have a big 40 gal tank with a pair of gargs and a pair of cresties!
I dunno, I'd still be worried that the males may go at it.