If it's because you have an assembly with references an older version or something you can do assembly binding in the web.config to get it loading properly.
Mmmm i need to develop 2 sites (diferent project).
The first project i need to use telerik version 2007.
in the second proyect i need to use the new version.
every project is for different customer.
But I need to have installed and develop in the same machine.