Your I.T. group has finally succumbed to your begging and pleading for a second monitor, but when you open two Excel spreadsheets you find you can't get them both visible at the same time, even though Word and PowerPoint behave nicely. Are you going crazy? Maybe, but this one is Microsoft's doing. But there is a solution.
Either hold the Shift key while left-clicking the Excel icon in your task bar, or right-click that icon and left-click on "Microsoft Excel". Both methods will launch a separate instance of Excel, allowing you to put them on separate monitors.