I assume you mean:
"I have a solution, with two projects, Test1 (which contains a control) and Test2 (which contains a form). How do I display the form from Test in the control in Test1?"
Because if they are both in the same project then you can just write
FormInTest2 frm = new FormInTest2();
frm.Show();
1) Add a reference to Test2 in Test1 (In solution explorer, right click "References" and select "Add Reference" - follow the dialogs from there)
2) At the top of the Test1 code file add the line
using Test2;
3) Add the code:
FormInTest2 frm = new FormInTest2();
frm.Show();