What storage options are specified for these fields? Can they be defined as fields with global data storage?
Are you trying to show all records where a date in a third date field falls on that interval?
When doing a month to month search, you'll also need to specify the year.
Describing the process in general terms, if you enter find mode and specify search criteria as a date range: 1/1/2012 ... 1/31/2012, this can be used to find all records where the date in a specified date field falls in that interval. This can also be done with relationships and portal filters, though the syntax will use inequality operators instead of the date range with "..." .