Type in TextBox after referencing a different Textbox

Perhaps code like this would do what you want, assuming that the + in the OP indicated concatenation rather than addition. This approach could be adapted to numeric addition.

Private Sub TextBox1_Change()
    TextBox2.Tag = TextBox1.Text
End Sub

Private Sub TextBox2_AfterUpdate()
    TextBox2.Text = TextBox2.Tag & TextBox2.Text
End Sub

Private Sub TextBox2_Enter()
    TextBox2.Text = Replace(TextBox2.Text, TextBox2.Tag, vbNullString, 1, 1)
End Sub