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