Touchy subject and it varies board by board how much should be tolerated.

Here's an example...

PTSC is sometimes called The Sister Company of EDIG. Many posters on both boards own both, move $ back and forth, state that they have, and they use the same screen name. Both boards tolerate this, unless someone bashes one for another.

IMO, if the poster is commenting negatively about one, and promoting the other in the same post, a warning should be sent via PM, and if it continues, flag the next post.