Well... they sometimes do go on exact parallel... but taking into account that xxxHOLiC releases erratically once or twice a month, contrary to Tsubasa's weekly publication (when they aren't taking constant breaks that is...)...

We'll probably get at least 5 more chapters of xxxHOLiC wraping Watanuki's story versus 10 to 15 chapters of Tsubasa wrapping up the battle with Fei Wong Reed and Syaoran's reunion with Sakura.

I also wish to see more of CCS Sakura and Syaoran...

Lets see... the series is finishing... so I guess its safe to assume this 'main' characters of other series never appeared on Tsubasa:
- Suu from Clover
- Kobato from... Kobato
- Main guys from Clamp Campus Paranormal Investigators

I don't remember who else... this very chapter finally added Hikaru.

We've seen Kamui and Fuuma (X), Akira (20 Menso ni Onegai), the Detective Trio (Clamp Campus Detectives), Chunnyan (Legend of Chunyan?), Chi (Chobits), Misaki and Hikaru (Angelic Layer... although the Deus only in backgrounds), Sakura (CCS... obviously...), Watanuki (xxxHOLiC) and probably some more I don't remember...

I need to buy this manga...