Open your first instance of Excel, and then right-click the Excel icon on the Desktop taskbar. Hold down the "Alt" key and select "Excel 2013" from the pop-up menu.

When opening multiple Excel files in their own separate windows, keep in mind that each window represents a separate instance of the program, and system resources used will multiply accordingly.

You can open multiple Excel files by making some changes in the way you start Excel. Don’t double-click on the workbook to open it, instead right-click on one of the files you want to open.

The Online Tech Tips blog explains how to use a registry hack to add an "Open in New Excel Instance" item to your context menu—which will open your spreadsheet in a separate copy of Excel, so ...

Q: When I open two Excel files in two instances of Excel and display them on two monitors, I find that I cannot then copy formulas back and forth between the files—only the formula’s calculated value ...