UserControl1.Controls.Add(UserControl2) - replace typenames with real instances
If these are UI controls you should be able to add one to the other by dragging them onto designer surface.
This should do the trick
UserControl2 u2 = new UserControl2
UserControl1 u1 = new Usercontrol1
u1.Controls.Add(u2)
It is just the same as if you're adding a button or textbox.