Unlike Word, PowerPoint and OneNote 2010 it’s not possible to have multiple authors working on a document at the same time in Microsoft Excel 2010. However, if you have access to the Office Web Applications – where it is possible to edit Word, PowerPoint and Excel documents via a browser – there is an alternative available.
The Excel Web Application has a feature known as Collaborative Editing – which is basically the co-authoring offered in Word, PowerPoint and OneNote – allowing multiple authors to work on a spreadsheet at the same time. Unlike co-authoring in the client, edits by other users working on the document are not applied when the document is saved, rather the document is updated as the changes are made.
There’s more info on the Excel Team Blog detailing how collaborative editing works and some example scenarios of how it could be used. There’s also good news in that this functionality will hopefully be available in future desktop versions of Office.