Io farei così

codice:
    Dim sFile As String = "C:\MyDoc.txt"

    ' scrivo
    Dim s As String = rtb1.Rtf & "|" & rtb2.Rtf
    Dim sw As StreamWriter
    sw = New StreamWriter(sFile, False)
    sw.Write(s)
    sw.Close()


    ' leggo
    Dim sr As New StreamReader(sFile)
    Dim words As String = sr.ReadToEnd()
    Dim split As String() = words.Split("|")
    rtb1.Rtf = split.ElementAt(0).Trim
    rtb2.Rtf = split.ElementAt(1).Trim