It shouldn’t be this problematic, but i must say i’m not familiar with Modo pipeline.
Try this; use a single UV channel with no overlapping faces, for the sake of testing. Import a couple of meshes like that and see if they work properly. If they do, then the problem is related to creating and keeping the second UV channel in Modo. If they still dont work then post the screenshot of your meshes UV layout inside the static mesh editor please.