I'm wondering how you have two versions with the same serial number? Volume license maybe?
In the world of single licenses an upgrade from CS4 to CS5 would get a new new number, at least in my experience. As far as installing multiple versions (but not differnt localizations, like US and ME, of the same version) no problem, and I often have multiple versions open at the same time for testing and answereing questions here on the forum.
No, that answers my question, thanks very much. The upgrade to CS5 was a new serial number, I couldn't understand why I could run 4 & 5 at the same time.
But I was pretty sure when I upgraded from 2 to 4, it was under the same serial number. Who knows. Obviously not me
Photoshop doesn't like to have two versions open at the same time, but ID is no problem.