Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim frmA As New FormA
        frmA.Show()
        Dim frmB As New FormB
        frmB.Show()
    End Sub

End Class

Public Class FormA
    Inherits System.Windows.Forms.Form

    Public Sub New()
        Dim btn As New Button
        btn.Text = "Button(&S)"
        AddHandler btn.Click, AddressOf ButtonClick
        Me.Controls.Add(btn)
        Me.Text = "FormA"
    End Sub

    Private Sub ButtonClick(ByVal sender As Object, ByVal e As System.EventArgs)
        MsgBox("FormA")
    End Sub

End Class

Public Class FormB
    Inherits System.Windows.Forms.Form

    Public Sub New()
        Dim btn As New Button
        btn.Text = "Button(&S)"
        AddHandler btn.Click, AddressOf ButtonClick
        Me.Controls.Add(btn)
        Me.Text = "FormB"
    End Sub

    Private Sub ButtonClick(ByVal sender As Object, ByVal e As System.EventArgs)
        MsgBox("FormB")
    End Sub

End Class

------解决方案--------------------
VB.NET code

Public Class Form1

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim frmA As New FormA
        Dim frmB As New FormB

        Dim panel1 As New Panel
        panel1.Dock = DockStyle.Fill
        panel1.Controls.Add(frmA)
        panel1.Controls.Add(frmB)

        frmA.Location = New Point(10, 10)
        frmB.Location = New Point(10, frmA.Location.Y + frmA.Height + 10)
        Me.Controls.Add(panel1)
    End Sub
End Class

Public Class FormA
    Inherits System.Windows.Forms.UserControl

    Public Sub New()
        Me.BorderStyle = Windows.Forms.BorderStyle.FixedSingle

        Dim btn As New Button
        btn.Text = "ButtonA(&S)"
        AddHandler btn.Click, AddressOf ButtonClick
        Me.Controls.Add(btn)
        Me.Text = "FormA"
    End Sub

    Private Sub ButtonClick(ByVal sender As Object, ByVal e As System.EventArgs)
        MsgBox("FormA")
    End Sub

End Class

Public Class FormB
    Inherits System.Windows.Forms.UserControl

    Public Sub New()
        Me.BorderStyle = Windows.Forms.BorderStyle.FixedSingle

        Dim btn As New Button
        btn.Text = "ButtonB(&S)"
        AddHandler btn.Click, AddressOf ButtonClick
        Me.Controls.Add(btn)
        Me.Text = "FormB"
    End Sub

    Private Sub ButtonClick(ByVal sender As Object, ByVal e As System.EventArgs)
        MsgBox("FormB")
    End Sub

End Class

------解决方案--------------------
注意这两处：
1、Public Class FormA
   Inherits System.Windows.Forms.UserControl


2、Public Class FormB
   Inherits System.Windows.Forms.UserControl