That appears to be the consensus, but I'm not sure if this is based on actual crossbreeding results, or just something people assume. If not on the same locus, it would very well be possible to breed a Bull which is homozygous both for Trumbower and Stillwater. It would probably result in an unusually bright colored hypo, perhaps even very similar to an albino. But as long as nobody reports anything on this, who really knows?
-----
I'm Dutch. Somebody shoot me.