If the report layout really is as smple as you have posted ... AND you ojnly need records where both BUY and SELL exist ...
Open one of the reports and model the fields.
OPen the other report as an EXTERNAL LOOKUP to the first report. Monarch will interpret it as a space separated 'database'.
Make the link using the CUSIP and ACCOUNT fields. Read in the But or sell field (depending on which file it is) or just set a flag to say there was a record to lookup.
Filter for the flag or the combination of a record with both buy and sell fields populated.
If it is a little more complicated and the direct lookup fails, process one of the reports to give you a valid external lookup table first and then link if as a lookup to the other report as described above.