Hi Frank,
Are you ganging the two on a common shaft?
Counter-rotating? it actually matters.
If they were closely coupled counter-rotators, then the diameter would be almost exactly the same.
Put them far apart, and the diameters would need to be maybe 87% of a single prop.
why? power is proportional to diamter ^5 for all other things equal, so 1/2 power absorbed is 87% diameter.
If they don't counter-rotate, I have no idea, especially as the effective pitch of the back prop will be reduced by the spiral airflow from the front one.
for what little it helps,
Dean

Frank is such a trend setter.  For general flying, it may not be an issue, but if you wanted to get a stunt run out of the deal, would there be enough battery on board to swing 2 props for that long? 

Additionally, on a common shaft, would you want to offset the props so that they appear as 4 blades when viewed from the front or rear or would lining them up be better?

So many things to think about.