In my opinion, you only need one version of Sonar on your machine. If you really want stability, uninstall everything, delete registry keys, delete folders etc, and just go with X2 with the X2a patch. I believe there is a step by step uninstall on this forum somewhere that will totally take it off your machine.
Though Sonar shares a few things amongst versions, X2 is installed in it's own folder and sort of does it's own thing like the other Sonar versions. The only thing you MAY get an issue with is plugins from newer versions. Or...once you uninstall older versions, those plugs will no longer appear.
So if you need the older plugs, you just do a custom install of that version of Sonar that contains those plugs and you can add them to your Sonar X2 experience.
To be honest with you, the only reason I keep a few versions of Sonar on my machines is in case there is a problem. For example, 8.5 is still my work horse. So it stays on my machines no matter what. If there is an X3 and it does surface, I'll probably remove X1 as I like X2 better and so far, X2 is proving to be pretty rock solid for me. I had my first crash last night and haven't seen one in months. It's always Kontakt that does it though, so I can't blame Sonar. Kontakt and me have never gotten along. I can't wait for something to come along and smoke the things that I need it for so I can uninstall it and burn it.
But my 3 versions are only here if there is some sort of compatibility issue. I save my projects with the name of the program used in the title so I cut down on possible "version" compat issues. On any pc, less is more if you can do things that way. :) I've only had one instance where a newer version of Sonar messed up an older one. It was a nightmare to fix, but I nailed it. But I've not seen anything since then and that was some time ago.
-Danny