Only one version of Opus can be installed on the same system, since they are both installed in the same place (and also need to register their COM objects).
Using a pair of configuration backups might make sense, so then you can flip between two configurations.
Exporting Opus 10 and your old config to a portable USB copy (if you have the USB add-on) is another way you could run Opus 10 and your old config on the same machine as Opus 11. (You still wouldn't be able to run both simultaneously. You'd need to fully exit one copy before running the other, otherwise running Opus again when it's already running would just tell the running instance to open a new window. To run two copies simultaneously on the same hardware, you'd need to use a virtual machine.)