You can open separate Excel instances by holding ALT when starting it up (second time onwards). Not quite the same thing as you want, but think the undo behaviour you describe makes sense if you're working in two sheets that are linked in some way.
Can you elaborate on this? It sounds like it would solve my "I can't open two files with the same name" problem too, but I hold down ALT and double-click my second file in explorer and I get the "Properties" dialogue for that file. If I instead hold ALT and click on Excel itself, it opens a new window but it still seems to be linked to the same process (and gives the same error when trying to open the second file of the same name).
This should work on windows - I'm not sure about Macs: you press and hold the ALT key, then right-click the Excel icon in the Windows taskbar and click the Excel icon above the taskbar, but keep holding ALT down. It should pop up a window saying "do you want to start a new instance of Excel". You can then open your (same named) file in the new instance.
