You didn't mention a DLL before.

Anyway, if you have two UserControls in the same project (or same solution of one project references the other), then you should be able to add an instance of one UserControl to the other.

Did you build the solution first? As you seem to know judging from your first post you need to build the solution before any controls show up in the Toolbox.

You are looking at the design surface of the UserControl, right?