Co-Author Excel Spreadsheet

SergeiBaklan thanks for reaching out! I have a Microsoft 365 subscription and have downloaded the desktop apps to my Windows 10 computer. I rarely use the Online version and save all my files to my computer that are backed up to my OneDrive account. The Excel file in which I am speaking of is saved as a .xlsm file and when I open it up from my computer AND open it up in the OneDrive at the same time THEN and only then can I see when my partner is online and making changes. But for some reason, I have to have BOTH files open at the same time. It will NOT work if I only have one or the other open. It's strange, I did try sharing the file from BOTH the desktop file and the OneDrive file but both come up with the same results. I have tried backing this file up to Dropbox but that didn't seem to make a difference. My partner does share my Microsoft so that isn't the issue either. I'm not sure why I would have to open the document up in OneDRive from a browser just to see each other and co-author a document? From what I have read, We BOTH should be able to open it up from our computers ONLY and not only see each other but co-author the same document that is being shared. Not sure what I am doing wrong?