Ok, the main form indeed looses focus, but why would you want to hide a modal dialog?
Common practice is to gather input from Form1 close the dialog and then open Form2 and gather its input, and then close it to return to modal-less MainForm.
Edit: anyway in your form1 button click event just switch owner to your main form
this.Hide();
Form2 SampleChild= new Form2();
SampleChild.DisplayDialog(MainForm);
VB.NET to C# http://www.developerfusion.com/tools/convert/vb-to-csharp/