Answer to 1st question - Yes, you can connect both rt and lt channels to a dual VC speaker... OCCD has valid comments regarding sound...
Answer to 2nd question - In the car audio world (some times la la land!) front stage rear fill is the way to go... If you can fit two nice 4" drivers in the dash and the 6 x 9's in rear deck would be the easiest way to go... I have seen a few 6 x 9 dual VC speakers, but have not seen 4 x 10 dual VC speakers...
HTH,
George