Razor,
I would do a complete backup first. A disk image backup would be ideal, in case if anything went wrong, it's easy to restore back to the starting point.
Installing X2 to your system together with your existing Sonar 8 should be no problem. I still have X1 in my system, though I seldom start X1 nowadays.
You do not need to install soft synts that requires additional serial number activation that you may already have (Dim Pro, Rapture) as you probably have it upgraded and with additional sound libraries.