Sorry, I tried this but it didn't work. When I Alt-Tab between open worksheets the sums of selected cells still disappear.
Things I've found that do work are below but they're not very elegant because you get a "PERSONAL.XLSB is locked for editing..." prompt on startup for all three options:
- Option 1 from MS TechNet below - holding down the ALT key and right-clicking the Excel icon on the taskbar.
- Option 2 from TechNet below - entering excel.exe /x from the command prompt.
- I can also create an icon on my desktop using the /x switch to run a second instance. (Be sure to add the /x outside the quotes in the target box.)
I'm sure MS will fix this eventually
https://social.technet.microsoft.com/Forums/en-US/4ce99c82-2d8a-47ac-bc0c-3fc469c9e6c5/excel-2016-separate-instances-seem-to-be-connected?forum=Office2016ITPro
In Windows 8, one way to start an additional instance of Excel is to press and hold the ALT key, then, without releasing the ALT key, right-click the Excel icon in the Windows taskbar and click the Excel icon above the Unpin option.

Continue holding down the ALT key until you are asked whether you want to start a new instance of Excel.

Click Yes to get a second instance of Excel up and running.
In general, anytime Excel is starting up, holding down the ALT key triggers the above dialog asking about a new instance.
- Start a second instance with command line
There’s also a switch for booting Excel from the command prompt: /x. In Windows 8, press the Windows key and X key at the same time and then select Run.

Type excel.exe /x, and then click OK. This
should launch a new instance of Excel.