Use the -home flag to set another database location for the second copy. syncthing -help ist your friend.
But maybe rsync or something fits better here…
Two instances with separate -home dirs should work here, although you’ll need to point one of them to the actual address (127.0.0.1:22xxx) as local discovery will not work within the same host.
Something like unison is probably far more efficient though. It’s a much easier problem to solve, syncing two directories that are both accessible to the same program, so about 90% or so of the work Syncthing does is unnecessary here.