Hi acj
MYOB Business will only produce sales reports based on the individual period you are running it for as a single value (or transactions that make up that value). The system doesn't have the ability to produce sales reports that will provide a comparison between periods.
Should that comparison report be required, you would be looking at running a report like the Customer sales (detail) for the individual periods, exporting it to Excel, and combining those reports within Excel. Alternatively, we do have third-party apps that may be able to extend that functionality and offer that reporting. Check out MYOB App Marketplace for a full range of apps available.