dgowans,
You can have 2 major version installed on the same machine, but you must be very careful to keep them in separate install folders. I actually run anywhere from 2-3 major versions (including Betas). I would recommend for your regular users that they just have 1 version installed - unless you need others for working with other parties that, say, run on a previous version. You also need to take care that your toolbox data is kept separate from major versions. All in all, I would recommend just sticking to one major version if you don't need 2 or more.
Here is my outline for updating users here from major versions...
1. Backup Data
2. Turn off AV software
3. Un-install previous version of SolidWorks (including all other programs)
4. Delete the install folders (all of them)
5. Clean the registry
6. Reboot
7. Turn of AV software
8. Install the Newer Major version
9. Apply company standards via a reg file
10. Run CCleaner (clears temp file junk and invalid registry entries)
11. Defrag