We may be talking about two different things, but unless it's a misunderstanding I think I'm going to disagree. I think I can open workbooks in two different instances of Excel—I've seen it happen, though I forget how I (accidentally) did it—but normally they're all in the same window and I can get from one to the other by hitting <Ctrl-Tab> and <Ctrl-Shift-Tab>. Try this: Minimize all the windows on your screen except the Excel windows. I expect you'll see just one.

The various Excel workbooks do appear in separate tabs in the task bar, and I can use <Alt-Tab> and <Alt-Shift-Tab> to get from one to the other if I choose. But there's a checkbox in the Excel options, advanced, in the Display section, entitled "Show all windows in the Taskbar"—I turned it off a couple weeks ago so it's fresh in my mind—that eliminates even that. So now even my <Alt-Tab> shows only one Excel window, and I have to use <Ctrl-Tab> to get from one workbook to another within Excel. I keep a lot of windows open, so it means less clutter for me.