I think so....
Clamp have always presented XXXHolic and Tsubasa together from the very very beginning. I think the first chapters of the manga ran at pretty much the exact same time as eachother. They are constantly releasing images and merchandise with the two series interwined, and both series are set in the same universe.
(Sakura and Syaoran appear in volume 1 of XXXHolic.... and Yuko and Watanuki appear in volume 1 of Tsubasa)
It could of been Clamp's intention from the beginning.
But man, what I wouldn't give for a movie ticket for that double header.