That not Strictly true, Sorry!
With out getting into reams of theory, there come a point ware swirl increases the effective AOA of the back blades. at which point the front blade start wind-milling. Contra props are not simple.
Have also been there done that.
See post http://www.rcgroups.com/forums/showt...246407&page=77
Peak O/P of 1.6KW. 755 Watts off the front blade & 852 off the back, flying with same pitch. Ended up with more on the front.