In Organisations using OneDrive, two or more people can edit a word document concurrently. To do this, 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.

See: http://office.microsoft.com/en-us/sharepoint-foundation-help/document-collaboration-and-co-authoring-HA102772333.aspx

Even so, it's important, if not essential, that users learn to use Word's Styles effectively and consistently. All too often, ham-fisted users stuff around with document formatting, overriding paragraph Styles, etc. to make them look like something they're not, then wonder why the document becomes harder and harder to maintain.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]