easiest way to do simultaneous builds with different options from same dir?

[Britton Kerin]

I'm frequently rebuilding and uploading both ends of a tx/rx system
and it's just slow enough to be annoying.  I'd like to rebuild the Tx
and Rx ends in parallel but they need different build options for the
same sources.

I'm guessing the easiest approach is to do cp -r or so from a script
and run make from there, and the tree is small enough that that will
probably work ok, but I'm curious if anyone has a trick to do it in
place somehow to avoid the cp and full rebuild of everything?
Something like automagically injecting $$ into all non-phony targets
or something?

Thanks,
Britton