how do I display 2 different spreadsheets in a split window?
Split
Divides the current window at the top left corner of the active cell.
Current is operative word, from Help. AFAIK you can’t.
You can manually arrange the windows to be stacked or side-by-side. For stacked you might prefer to use Single Toolbar interface for more space, View > User Interface > Single Toolbar.
If you have only the two spreadsheets open and you are using Windows, you can right-click on the taskbar and select Show windows stacked
If you are using Windows grab one by its title bar and drag it to the left until your cursor touches the edge, release and just click the other window and they will be side-by-side with half the screen each.
Thanks. That will work.