程序代码：

Public Class Form1
    Const EM_LINESCROLL = &HB6
    Private Declare Function SendMessageBynum Lib "user32" Alias "SendMessageA" (ByVal hwnd As Integer, ByVal wMsg As Integer, ByVal wParam As Integer, ByVal lParam As Integer) As Integer

    Public Function GetFirstLine() As Integer

        Dim point As System.Drawing.Point
        point.Y = 1
        point.X = 1
        Dim start As Integer = RichTextBox1.GetCharIndexFromPosition(point)
        GetFirstLine = RichTextBox1.GetLineFromCharIndex(start)
    End Function

    Private Sub RichTextBox1_VScroll(ByVal sender As Object, ByVal e As System.EventArgs) Handles RichTextBox1.VScroll
        RichTextBox2.ScrollToCaret()
        SendMessageBynum(RichTextBox2.Handle, EM_LINESCROLL, 0, GetFirstLine)
    End Sub
End Class