Or have both clients upload different parts to the server over the course of the transfer, and optimize for the client with the highest upload speed to do most

Or have both clients upload different parts to the server over the course of the transfer, and optimize for the client with the highest upload speed to do most of the heavy lifting (this assumes you want the server to definitely have a full copy first). Sounds like I'm saying make the server another peer as you may get even more transfer speed sending half the file to the server, then the other half from the server and sender to the receiver - depending on which you want to have the complete file first/faster/at all (more general).