I think it's easy to do when you first start out. I know when I started reading xxxHolic I had no interest at all in Tsubasa. That changed, when the crossover elements became more apparent. Now, though, I really think it's impossible to understand one story without the other, you'd miss out on too much.

Like, if you didn't know what was going on in Tsubasa? You would be so confused as to why Yuuko is currently being eaten up by black goo. Not that, you know, I'm entirely sure why she's being eaten by the Blob in any case, but not paying attention to Tsubasa would bring the level of WTF up above the stratosphere.