Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

Dim DialogForm As New Form2
DialogForm.ParentForm = Me
DialogForm.ShowDialog()

End Sub

[Form2]

Private Sub Form2_Resize(ByVal sender As Object, ByVal e As System.EventArgs) Handles MyBase.Resize

Me.ParentForm.Visible = Not (Me.WindowState = FormWindowState.Minimized)

End Sub

と、最小化の際にForm1を非表示にすることで対処しました。