per ora ho risolto usando una trackbar:
codice:
Public Class Form1
Private Sub TrackBar1_Scroll(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TrackBar1.Scroll
tb1.SelectionStart = TrackBar1.Value
tb1.SelectionLength = TrackBar1.Value
tb1.ScrollToCaret()
tb2.SelectionStart = TrackBar1.Value
tb2.SelectionLength = TrackBar1.Value
tb2.ScrollToCaret()
End Sub
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
TrackBar1.Minimum = 0
TrackBar1.Maximum = tb1.Text.Length
TrackBar1.Value = 0
End Sub
End Class