Hello Tom Thomas,
You can't have both installations simultaneously, however you can have the SL4 installation which will add the controls automatically to the VS2010 toolbox. Then you can download one of the following two:
First one contains exactly the same files as the installation with the Demos, help etc... , the second contains only the assemblies. Then you need to follow the steps from this help article
to add the SL3 controls to the toolbox. Note that the article is a little bit old and with the official release of VS2010 you no longer need to copy the assemblies in the Public Assemblies folder.
If you have further questions please let me know.
All the best,
the Telerik team
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