Milne-Thomson is available on Google Books, but the preview stops several pages short of the desired item! Here, however, is not a proof, but a test.http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930081810_1993081810.pdf
"It can be seen from the foregoing discussion that the
biplane has no useful counterpart of the monoplane
aerodynamic center. For this reason, biplane problems
are best solved by proceeding directly to a solution of
the forces and moments."
NATIONAL ADVISORY COMMITTEE
FOR AERONAUTICS...
Milne-Thomson gives a mathematical treatment of the slipstream in "Theoretical Aerodynamics, 4th edition, Dover, pp 235-242. Yes, the propellor induces radial, axial, and TANGENTIAL motions to the slipstream, and is thoroughly described.