Tech Byte:
Side by Side
Need to compare two
Excel workbooks? Forget toggling between them or trying to
resize their windows manually—Excel will do it for you,
automatically.

Simply open both
documents and choose Compare Side by
Side from the Window menu. You can choose a vertical
split with the worksheets in a side-by-side view, or a
horizontally split above/below view. Open the Window
menu and select Arrange...

Bonus:
You can synchronize scrolling, so the two
views move in sequence.