What version of SharePoint are you using? If you have Enterprise, you can look at using Excel Services, which is very good for multiple, side by side edits.
Failing that, what you're asking for isn't possible. When your users open the file, it'll be downloaded to either the SharePoint Drafts folder (if enabled) or the local temp folder. Saving the changes made to these files simply uploads the new
file to the server. This operations overrides the previous one. It doesn't perform any version checking between whats online and what's being saved. All your users will simply override each others work, if no form of document control or mandatory
checking is enabled.
Steven Andrews | SharePoint Professional | http://www.twitter.com/backpackerd00d | https://baron72.wordpress.com/