The desktop Office apps are more powerful than their Online counterparts, but the current release doesn’t support the real-time co-authoring that helps us collaborate. Luckily, you can opt into the “Fast Insiders” version that enables this key feature. While it’s technically an unsupported release, feedback from users so far is that it’s stable and gets rid of those “locked for editing” errors that happen when multiple users use the regular desktop apps.
Get instructions on signing up direct from Microsoft’s support site: https://products.office.com/en/office-insider?tab=Mac#Tabs_section