Pointless idea, each team will thrash the other one when playing their own code.
But a hybrid game now may work.
Main rules :- 13 a side, get rid of the 2 flankers. Loose the pointless line outs Get rid of the ugly rucking and mauling and play out a number of tackles ( say six ) 4 points for a try 2 for a penalty 1 for a DG Maybe just maybe make the scrums contested.