But how I gona to make sure every number from 1 to 8 are come out in a pairs?
You would just use the method randomInt twice to get a pair. If you mean that you wouldn't want a duplicate value in your pair, then you would keep calling it until you didn't have two of the same values in your pair.
For Each ctrl As Control In Me.Controls("pnlMainPanel").Controls
If ctrl.GetType Is GetType(System.Windows.Forms.Panel) Then
For Each subCtrl As Control In ctrl.Controls
If subCtrl.GetType Is GetType(System.Windows.Forms.TextBox) Then
If subCtrl.GetType Is ...