They don't use the aerospace phenotype, and FM:Crusader Clan states that "Unlike many Clans, failure in the initial Trial of Position does not immediately relegate an aspiring Ghost Bear into the ranks of lower castes. Rather, the cadet is shifted to another warrior subcaste, his or her placement based on aptitude tests, in the hopes that the cadet may test out more positively there."
So its possible to find Ghost Bears that have trained for one, and gotten into the other branch.
I'd also expect to find IS special forces that have cross-trained. Obviously not all of them, but some sure.
Heck, pretty much all of the early ProtoMech Warriors were cross-trained, since they were failed aerospace pilots, who then transferred into the ProtoMech training program.