If you need to see two workbooks at the same time, you can arrange them using the Windows menu. Go to Window > Arrange and choose the best option for how your data is laid out and what you want to see.
I find this most useful when I want to see two different parts of the same workbook, either different sheets or even the same sheet. One such example is a net worth spreadsheet that I inherited. I’ve made a simple reproduction of it below. The summary page starts in A1, but the details portion is off the screen, presumably so you could add and delete rows and columns in one without affecting the other.
To view the summary page while I’m entering the details, I first create a new window for the workbook using Windows > New. Once you have a new window, you’ll notice their title bars look like MyBook1.xls:1 and MyBook1.xls:2 indicating two windows for the same workbook.
Then using Windows > Arrange > Tiled, I can scroll each window where I want it. I can enter details in the details part of the worksheet and see how that affects the summary part as I do it.
I think we can agree that a $200 couch and a $2,000 TV are all anyone really needs.