Your description suggests a network problem, with the networking software losing track of the fact the file is already open.
That said, you might find it useful to store documents intended for shared editing in a OneDrive folder. This is designed to allow two or more people can edit a word document concurrently.
Once you have OneDrive operational, click OneDrive in the header of a SharePoint or Office 365 site, then click the ellipsis (…) to the right of the entry. Then click ‘Share’ and invite the other user to view or edit the document, and click ‘Share’ once more. To add to the list of people already sharing a document, click the ‘Shared with’ icon in the ‘Sharing’ column and invite the additional person to view or edit the document, and click ‘Share’ once more.
When someone invites you to share a document, you’ll typically receive an email notification with a link to the document. Clicking the link opens the document. If you have edit permission, your changes are automatically saved.
For more info, see:
http://office.microsoft.com/en-us/sh...102772333.aspx