Is it common for a writer to write for more than one demographic? CLAMP writes for more than one demographic of manga--Cardcaptor Sakura is shoujo while Tsubasa: Reservoir Chronicle is shounen and xxxHolic is seinen.

Masashi Kishimoto, the author of Naruto, however, has only written for Weekly Shōnen Jump, implying that he has only written shounen.

Is it common for writers/writing groups to do what CLAMP does and write for more than one demographic or is it more common for them to do what Masashi Kishimoto does?
It is common for an author to experiment with different genres, art, and media. But at the same time, it's also common that the same author uses different names for different demographics.

You should distinguish between the writer and the pen name he/she uses, and between individual authors and groups. CLAMP is a collective and they're changing its components periodically. On the other hand, Masashi Kishimoto is an individual author (that it doesn't mean that he hasn't any assistant).

Pseudonyms are common in this industry where we can have an individual author Sumomo Yumeka writing yaoi with this name, seinen manga using the pseudonym Mizu Sahara, and writing shoujo manga as Sahara Keita. Any pen name is clearly associated with a single demographic in this case, but the author herself actually write for different demographics. It is the same with Ken Akamatsu (shounen) aka Awa Mizuno (hentai doujinshi), so it seems that where publishing constraints are very different, authors can use a pen name to differentiate their works.

Collectives and circles like CLAMP seems to explore different genres/demographics more freely with the same name, probably because the authority is shared among the members and the collective nature of the name is clear to readers.