I would say that it's more unrelated than 'related'. Yes, Syaoran and Sakura are the same, personality-wise, but, they no longer use the Clow/Sakura cards.
It's more of an evolution of a story line, if that storyline were to occur in a parallel universe. Because Tsubasa ties in with XXXholic, you have intervention from the 'Space/Time Witch', Yuko. If you're a fan of XXXholic, you'd know that, in several of the volumes released so far, Yuko mentions that she and Clow Reed did have some kind of 'relationship' together and, with his help, Yuko was able to create the Mokona duo. XXXholic has not gone into detail as to Yuko's relationship with Clow Reed, but, it's likely that future chapters will explain this...especially as Yuko is now concerned that a formidable presence has returned to cause chaos.