Not sure if this is possible.
Due to a ridiculous TFS structure, I had to develop the Silverlight apps in independent projects.
Is there a way to setup a usercontrol so it can just be dropped into the 2nd project? Or do I need to move all of the related code/classes over to make it work?
The tools menu can show any controls from any referenced DLLs. Create any reusable controls in separate Silverlight Library projects. You just add your control library project references to each Silverlight solution, or directly into the toolbox if you prefer.
I would actually suggest you start treating your own controls as a third-party control library. Keep them in well organised and well-named projects following common naming standards like:
companyname.category.controltype.versionnumber
Assuming you have created robust self-contained code, you only need to reference the dlls and not the original projects. This will speed up build times.