Well, kurogane also comes in as one of the main characters in Tsubasa, although him being in XXXholic would explain why the two manga seem to be so closely linked. I've only seen the 3rd epidsode, when they're in the kansai world, but he should already be there. *ponder* is his name different in the anime? He's the guy in all black who has the dragon familiar in the Kansai world.
i think i read somewhere that clamp intended xxxholic to be a "parallel" anime to T:RC. basically, the xxxholic story runs in an alternate universe but along the same timeline as all the other clamp anime, and somewhere during the series, a crossover from T:RC happens, which is of course the first ep of T:RC. kinda neat idea methinks. but dont quote me on this info cos i remember reading it somewhere. duno if it was a good source. maybe someone who watches xxxholic could confirm?