I opened your database and could not find anything in it related to anything you said. I am going to answer the question differently.
When I had multiple combo boxes to contribute to the WHERE clause for a query, OR when they would contribute to the implied WHERE clause of a form's filter, I built a criteria string FIRST using VBA to concatenate the parts I wanted to use, then applied it as a separate step.
Since you are interested in TWO combo boxes but sometimes one of them isn't used, what I would do is use VBA to build the selection string before I tried to apply the combo selections. In a combo box, if nothing is selected, its .ListIndex is -1, so it is an easy test.
Code:
strCriteria = ""
IF Me.cboA.ListIndex <> -1 THEN
strCriteria = "[TransactionDate]= #" & Me.cboA & "#"
ELSE
IF me.cboB.ListIndex <> -1 THEN
strCriteria = "[TransactionDate] = #" & Me.cboB & "#"
END IF
END IF
IF strCriteria <> "" THEN
Me.Filter = strCriteria
Me.FilterOn = TRUE
END IF
This snippet picks the A combo box in preference to the B combo box if both were used. You didn't tell us what to do if BOTH were used but that is just a matter of adjusting this logic to fit. Since I sometimes built dynamic SQL strings for use with a db.Execute operation, I frequently built the string separately and then activated it. But this method would work either for filtering or for building a new SQL string.