Changing ControlSource Binding

I have a form. The form is bound to an underlying query which is a left join between two tables. (SELECT ... T1 LEFT JOIN T2 ON T1.RID = T2.RID ....)

Now, what I want to happen is the value in T1.RID does have a match in T2, then I want a sequence of text controls to be bound to fields from T2, otherwise I want them to be bound to fields in T1.

I almost have it working. I have Form_Current() and T1_RID_Change() both calling a routine

sub BindControls()
if isnull(T2_RID.value) then
txtField1.Controlsource = "T1.Field1"
txtField2.Controlsource = "T1.Field2"
' etc for each field
else
txtField1.ControlSource = "T2.Field1"
txtField2.ControlSource = "T2.Field2"
' etc for each field
end if
end sub

The problem is that when I change the value in T1.RID, most of the times it does change the controlsource appropriately. But other times it lags. It keeps the wrong controlsource until I try changin RID again, and sometimes the text boxes are just blank, so I am not sure what is going.

Am I ultimately going to be able to make this scenario work?

Thanks,
David