I went through the calculation for the front shaft using an online java
calculator here: http://www.tech.plym.ac.uk/sme/desnotes/shaftMandT.htm
The attached screen shot shows the result of the calculator and would

indicate that the front shaft is indeed capable of taking the full
torque pulse of a 2 rotor, even if you add a significant fudge factor.
If you only ask it to take ½ the load?  Dual prop amphibian?
Decker, Michael

What stress level or material did you enter the program with?

Paul Lamar