How to open workbook in multiple Excel sessions?

Simple enough question but the answer seems to keep changing all the time 😞  So I start new instance of Excel365 by pressing down ALT but where-as I used to be able to open the same workbook I have in my principal Excel session "Read-only" in that second session, I now get "... open in another application.  Please close it and try again."  What does one have to do today to get a read-only second copy?