Yeah, if they were to meet the original Sakura, that would be in the world Yuko's in. However that's not saying it won't happen.
I'm curious if they'll end up in the world where Magic Knight Rayearth was set in! THAT would be interesting and that is where Mokona came from. Yuko says in the xxxHolic manga that she and Clow got the Mokonas while in another dimension... stands to reason that this Mokona could be the same one from MKR.
btw, Miyuki could very well be a plot point. CLAMP is famous for making the smallest things have great significance later on in the series. eg Yuki being hungry in CCS, Saoran kicking with his legs in Tsubasa, etc...
Actually, Yuuko didn't get the same Mokona... She worked with Clow to create a pair of them, black and white, after meeting "the powerful yet delicious creator of the universe"... She actually said delicious, and she even thought of giving them the name "Black Fillings" and "White Fillings" You won't find this scene anywhere except in CLAMP's website's albums. It's about the adventures of the two Mokonas long before Card Captor Sakura, Tsubasa, or XXXHolic kick in... There's so many interesting thing happening among Mokonas, Clow, Yuuko, Kero, and Yue (according to the album, they did just about everything together, playing, drinking, drinking, drinking, drinking, drinking, drinking, drinking... <--Blame on Yuuko for all the alcohol abuse)
Actually... it can very well be that CLAMP just add it in for the sake of adding it in... Like the time when Legal Drug Store has a 1-2 pages of interaction with Watanuki.