If the two Excel workbooks have the same name, you cannot open them at the same time. For this, either close one of them and then open the other or change the name of one of them.

Microsoft Excel and Microsoft Excel Online allow users to collaborate on Excel Workbooks with the help of co-authoring. Here's how to do it.

Like PowerPoint Live, which Microsoft introduced earlier this year, Excel Live allows users to access and share their latest Excel workbooks by using the Share tray in Teams.