You are into a zone with very little information. There was an RCModeller plan called the Super Libelle which was
a coaxial. The upper rotor was a different size and set to a different pitch.
There's not much info to help with. The suggestion to fly a similar model first (Al's auto I think) and learn
what works is probably the best advice you'll get. I'd expect that you will need a different top rotor from the bottom one, probably with more negative pitch. You'll be definitely doing lots of experiements. Who knows the first guess may be right on the
money and it will fly right off the board.
mickey