More likely than you might think, though the specific "difficulty", for lack of a better word, would be getting the two to have either a substantial enough difference in height or endowment size to allow them to cock-fuck each other. By and large, Val and Brock tend to prefer being similarly sized, and when more extreme size differences happen (which is hardly a rare occurrence I should clarify), they will tend to engage first in more "regular" forms of sexual activity, using the disparate size to forgo the use of magic to make things "work" (e.g. not needing to dislocate a jaw like a snake would to give a blowjob to one of their hyper sized cocks)!
That said, the two are very regularly large enough to make a mess of a city (and then some), but they're also nice enough to clean up afterwards!