But don't decide it is a major advantage for me, will there be any trouble going back to Sonar X1 - or will it be an entirely separate installation?

You wont have any trouble going back to X1.

XI icon will still be available to open X1. You just need to be careful if you uninstall x2 as you could delete plugins if the x2   plugins are installed in the same folder as X1.
I would choose different folders when installing the trial verion you can make them anything you like such as c:\sonarx2
and for the plugins for x2 put them in c:\sonarx2\plugins that way you will have no issue if you want to remove the trial.