Collaborating on password protected excel

Hi there, I am looking to collaborate with another colleague on a password protected Excel, however, the document doesn't open in the browser (I get this message: We're sorry. We can't open the workbook in the browser because it uses these unsupported features: Workbook encryption. Do you want to open the file in the desktop app instead?) 

And when 'opened in app' we are not able to work on it at the same time. 

Does anyone know how I would be able to work on this excel sheet at the same time as someone else, or is this not possible with password protected sheets? 

Thanks!