I recognize how important it is to you run both reports simultaneously, @retchison.

The filter to run the report for the first two weeks of the previous month is currently unavailable. It'll only show the first two weeks of the current month and the last two weeks of the previous month.

Also, the screenshot shared by my colleague above is for the previous year. Since it doesn't work, you'll want to export your first two weeks of the current month's report to MS Excel. Then, add an additional column for the first two weeks of the previous month. Filter the report to the later month for reference. I'd be glad to show you how.

1. Go to the Reports menu in your QuickBooks Desktop (QBDT).
2. Select Company Financial.
3. Click Profit & Loss Standard.
4. Customize the report base on your preference.
5. Click the Excel from the top toolbar.
6. Select either Create New Worksheet or Update Existing Worksheet. 
7. Click Browse and then find the workbook on your computer.
8. When ready, select OK. To open the report in Excel, you can also select Export.
9. Once exported, customize the report again for the previous month.

You can utilize this link for the detailed steps: Export reports as Excel workbooks in QuickBooks Desktop.

You can also look for a third-party app that lets you run both reports simultaneously. Hover to the App Center in your QBDT or visit our Apps for QuickBooks Desktop page for reference.

I'm also attaching this resource that can help you in managing your reports flawlessly: Understand reports.

Keep me posted if you have other questions running your Profit & Loss reports. You're always welcome in the Community space whenever you need help. Stay safe!