A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
If you happen to have dual monitor, then you could open each of them in a spearate Excel instance (start Excel twice) and drag each Excel window to its own monitor.
Alternatively, Open both files in Excel, select the tab you want to see on both and then select View, Arrange All. Select one of the options in the dialog that pops up.
Another way is to copy the relevant part of one sheet and then go to the other sheet, select the top-left cell of the same range and then do a paste-picture-link.