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 Exce

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.