This question is a follow up to my question about sharing resources between web applications, because I have not yet found a good solution.
I have a web application
Check this post for how to solve this issue.
The basic idea is to change your project to have a prebuild step to copy over the .ascx files to a subdirectory of the web application. Then just refer to those copies when using them. Of course you also need to reference the UserControls assembly as well.