You can add a split container to your mdi parent. Cut and place your navigator into one panel and open your form in the other panel.
To add your form into the other panel do something along these lines:
MyForm frm = new MyForm();
frm.MdiParent = this;
this.splitContainer1.Panel2.Controls.Add(frm);
frm.Show();
Remember howeverto also remove the form from the panel before closing it.