Sharing User Controls across projects is something everyone needs to do.
You create this great thing in Project A and need to use it in Project B.
The best way I have found to do it is with virtual directories in IIS, or sub-projects in sub-folders.
Basically you create one project as a control/class library, but provide no functionality in it, no web.config, no global asax, and NO “Application” in IIS. Take this project and stick it at inetpub\wwwroot\ControlLibrary.
Then in your other projects you create a virtual directory of “ControlLibrary” that points back to inetpub\wwwroot\ControlLibrary. All you have to do in your other projects is have a reference to the ControlLibrary.dll, or a project reference in one solution. Then just load the control with a Page.LoadControl() and pass in “ControlLibrary\SomeFolder\SomeControl.ascx”
Good Luck!