The easiest way to have two different copies of Scribus installed on OS X is to put one in the system Applications folder and one in your personal Applications folder. (With OS X, I don't think the operating system cares where you install an application, the Application folders being more of a suggestion than a requirement.)
As with the two replies already given, I had no problems with 1.4.x and 1.5.x being installed at the same time (quite a while ago).