You should be able to keep two files open in Mac Office whether it be Excel, Word, or Powerpoint. You can switch between them using the Command + Tab keys. You do not have to run two instances of the program.
But the OP wants to see both files at the same time, "split screen".
Bingo! While Vansmith's method does work (which I never realized, since I'd never tried it) this is the winner. Funny that I never investigated the possibility, because I do have reasons to use this feature frequently. Now that I've had my eyes opened, I'll make good use of it! One cautionary note: I had a devil of a time getting it to work until I discovered the reason was that one of the files I opened was protected. Changed that, and it worked!OK, I just tried it on the latest version of Excel. Open two (or more) spreadsheet/workbooks. From the top bar, pick Window/Arrange and then choose whether to view it split horizontally, vertically, cascaded or tiled.
Ok, but my question was in relation to the OP's question. I thought that it might be necessary to open the application again in order to see two files on the screen, because I've always used the method you mention to switch between open files. I was under the mistaken impression that only one window was viewable at a time due to the fact that it fills the screen. I usually don't use full screen mode, but with the "ribbon" I'm forced to if I want to see all the buttons. Lately I've taken to clicking the ribbon "off" or rather to "reduced mode" so I can see more of the file, turning the ribbon back on when I need it.Then he can use the method that "Vansmith" recommended. I was answering your question about having to run two instances of Excel.
Lately I've taken to clicking the ribbon "off" or rather to "reduced mode" so I can see more of the file, turning the ribbon back on when I need it.