That will not work, because the entire statment is evaluated before
execution.  A way around that would be to write a function to determine which
form is open, evaluate the dates, and return a true of false value.  Then,
change the criteria in your query to use the function.  The function will
have to be a Public Function in a standard module.  It might go something
like this:

Public Function IsGoodRevenueDate(dtmSomeDate) As Boolean
Dim frm As Form

   If Currentproject.AllForms("frmSelectRevenueDates").IsLoaded Then
       Set frm = "frmSelectRevenueDates"
   ElseIf Currentproject.AllForms( _
           "frmSelectRevenueDatesforSummaryRpt").IsLoaded Then
       Set frm = "frmSelectRevenueDatesforSummaryRpt"
   End If

   With frm
       If dtmSomeDate >= .txtBeginningDate And _
               dtmSomeDate <= .txtEndingDate Then
           IsGoodRevenueDate = True
       Else
           IsGoodRevenueDate = False
       End If
   End With

   Set frm = Nothing

End Function

Then Change your query criteria to:

     IsGoodRevenueDate ([DateFieldName])

DateFieldName is the name of the field you want to evaluate.