You cannot have more than one version of Telerik controls in the "Add Reference" dialog. This behaviour is by design in order to avoid version problems. Our RadControls for Silverlight installer removes any older Telerik references from the dialog and sets the new ones if you choose one of the "Visual Studio Extensions" subfeatures (the Visual Studio integration) in the installer.
I suggest you to use our upgrade wizard (Telerik | RadControls for Silverlight | Upgrade Wizard). It will set a reference "Hint Path" pointing the installation folder of the version you choose from the wizard dropdown. Upgrading your old projects (to the same version) will preserve their references when you install newer parallel version.
If you have more questions please contact us again.
the Telerik team