Dave,
As I understand it you are trying to gain some effeciency by putting the rear rotors in the slipstream of the forward ones as a make shift coaxial set up. I think that when the prop rotors are rotated down into forward flight mode they will be a good distance apart. From what I have read the coaxial loses the performance gain as the rotors are set further apart. I have thought about this arrangement myself and one possible solution would be to use the forward rotors as pushers pointing down mounted on a high wing and then rotating aft which would put the prop closer to the rear rotor. The rear set could be tractors pointing up on a low wing and then rotating forward. I'm not a CAD whiz so I can't draw it up but I have played with the idea on paper a bit and it seems to work out.
I think one of the reasons the tiltrotors are not as effecient is due to the load they are trying to carry with it. For a military machine or one that is being used for revenue this is important but for a personal air vehicle maybe not so much. Take a V22 and cut down the gross weight to just empty plus fuel and crew and the required structure to carry the lesser load and the disc loading goes down a bunch. With lower disc loading you need less power and therefore smaller engines etc. Things might start to look a bit more promising at this point.
Just some random thoughts about your idea for what they are worth (not much at todays exchange rate!!!).
Max