I just purchased and upgrade to DOpus 11 from version 9. I would like to keep both versions available for awhile. When I run the V11 installer, it indicates that it will replace V9. Is it possible to have both?
No, at least not with the normal HDD install. There are COM components used to integrate Opus with Windows and other apps, and they can only be registered in one place.
Before installing the new version, do a config backup. You can then switch back to the old version if you really need to at any point, and reinstall the backup.
Configuration will be imported from old versions into new versions (with a few exceptions that get reset), but not always in the other direction. (It sometimes works, but is harder to guarantee, since we can't change what the old versions does after the fact, of course.) A config backup made with the old version solves that.
See also: Your old toolbars are still there after you upgrade if you are worried about your toolbar setup. The new version will start you off using the new default toolbars, but you can switch back to your Opus 9 ones if you wish, or you can load both at once and copy things over to mix your custom buttons with the new defaults.