Responses
Honestly, you can read either series independently. They do intertwine and influence each other at points but not enough to put you out of the story. I'd say the best way to read it would be choose between xxxHolic or Tsubasa, then read xxxHolic Rei (which I don't think was completed, but read after the main series), and then finally Tsubasa World Chronicles (Which was the most recent sequel to release, nothing new yet since then)
I really want to read Tsubasa: Reservoir Chronicle and XXXHolic but i don't know where to start. Can someone give a link or idea where to start or perhaps a seqence or order what should read first?