I need some help on tracking changes on a richtextbox

Question

 Hello Everyone:

I have an edit form where I populate data after a search is done.  In the edit form I have two Richtextboxes

I would like to keep track of who makes the changes or edits the form, and I have the following code: and is working just fine:

Dim stamp As String = "Edited by " & LoginForm1.UsernameTextBox.Text & " on " & Date.Now.ToString

Me.RichTextBox1.AppendText(Environment.NewLine & stamp)

Me.RichTextBox2.AppendText(Environment.NewLine & stamp)

The problem that I am having is that I would like to only add the tracking  log to the RichTextBox that I am modifying.

Right now if I edit RichTextBox1, it will add the tacking log, but is also adding the tracking log to the RichTextBox2, and I have not touched RichTextBox2,

How can I make it to work to only add the tacking log to the one that I am touching?

Thanks for your help