To answer many similar questions, I wrote and article on form collaboration for .NET; I also covered WPF. I did not bother to cover Delphi (which could also cover Free Pascal with Lazarus library LCL), but as Delphi is predecessor of .NET, you can easily apply the same principles.
So, try to read the article and understand it; if you know Delphi, you can easily catch the idea of the fairly trivial solution, even if you don't know C#:
Many Questions Answered at Once — Collaboration between Windows Forms or WPF Windows.
If something is unclear, your follow-up questions will be welcome.
—SA