have you set the Main menu's isMdiContainer property to True? and the set the second form as a child form of the main one?
Public Class frmMain
Private Sub frmMain_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
IsMdiContainer = True
End Sub
Private Sub Form3ToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Form3ToolStripMenuItem.Click
Dim frmThree As New frmThree
frmThree.MdiParent = Me
frmThree.Show()
End Sub
End Class