Here is a followup, if you are interested in using Office365 more.
I confirmed that I can open and edit the same Excel file on two devices at the same time (my Mac and my iPad) if I just use the web (office.com) and close the file in the native Excel app on my Mac first.
If I have the file open in Excel on my Mac though it locks it and cannot be edited in the web or on my iPad, as reported in my original post.
So the secret of getting it to work “sort of like Google apps” is to completely stop using the native app it seems.
Synchronizing is much slower though. In Google apps you can literally see what cell the other user is in and get updates in real-time. In Excel via the web and the iPad, you can’t see what cell the other user is in, and while it does sync, it takes 30-60 seconds for the sync to come through.
I don’t know which is better as far as features or UI via the web at this point. Excel via the web does have some things that Google Sheets does not have. For example you can drag to select cells, and the file does open at the last row you were at. Those are pluses.
Also, with OneDrive and Office365 you have your actual files synced to your computer. With Google Apps you don’t.
Anyway, it does seem if you are careful not to leave your OneDrive documents open in the native app it does work. However if you leave it open in the native app you can only view on the other devices, not edit.