Hi Malvaro-
One way you could retain MasterTable as control source for TextBox1 and capture the state of the checkboxes in the TextBox control is to use VBA code. Some sample code that you can try (perhaps with a little modification needed; should work if your textbox is named TextBox1, and your checkboxes are named CheckBox1 and CheckBox2) with your form is provided below. Fire up the Visual Basic Editor either through the menu/ribbon or Alt-F11 and place the code in the form module for your form.
Hope this helps.
Code:
Private Sub CheckBox1_Click()
CheckCounter
End Sub
Private Sub CheckBox2_Click()
CheckCounter
End Sub
Sub CheckCounter()
If CheckBox1.Value <> 0 And CheckBox2.Value <> 0 Then
TextBox1.Value = 2
ElseIf CheckBox1.Value = 0 And CheckBox2.Value = 0 Then
TextBox1.Value = 0
Else
TextBox1.Value = 1
End If
End Sub