How can I support both floor joists and roof trusses while I replace wall framing below?
Assuming full access to both sides of the wall, I'd look at installing a temporary beam or wall tight to the inside face of the wall framing. It's a safe bet that you'll catch some of the truss heel there. It won't take much to carry the load for a short term, and you can rework the framing from the outside. 

If the truss heels don't extend inside the wall, you may need to leg up from the beam/wall to the trusses. Simply screwing legs to the sides of the trusses would probably do, depending on the roof structure as a whole. 

All this depends on factors that I don't have information about, though, some of which I've mentioned.