I feel like you should be able to combine panoramas from multiple source projects into a single tour. Can they not be tied together in the tour by their positional data alone? Our current project has portions in Revit and Rhino and, as long as the two models are matching up spatially in reference to the their respective origins, I don't see why this shouldn't be possible. You can get the panos in the same project folder by using the new update feature, but you just can't create a tour with them.
so close! so frustrating!