On the Multidevice Extension script, there's a multi-select box for "Additional phones to ring", which is good for adding softphone logins to an extension, for example.
One thing I've discovered is confusing some of my admin folks when they set up new phones is that they miss the word "additional" when choosing the extra phones there (often when setting up a softphone at the same time that is really all that should be added there). Although this is relatively harmless (Asterisk just ignores it with a "not dialing SIP/265 because it's already being dialed") I figure it might help get the point across if the primary device for that extension isn't in the list to begin with as a choice. However, because of the generic way scripts work, I realize this wouldn't exactly be straightforward to implement either. You'd either need a new Phones/Lines type that excludes the one listed in the previous Phones/Lines argument, or you'd need an argument to the type to tell it to do so, and I can't think of a good way to pull it off that wouldn't be even more confusing. :) But I just thought I'd throw it out there.