Multiple co-authors can edit the same plan at the same time, but they can not edit the same page. When one user is in a plan on any specific page, that page is locked. If a second author arrives on that page, they will get a warning that the page is unavailable for editing. Others can still view it, but they cannot make any changes.
A page is immediately unlocked once that user goes to another page in that plan. When the first user navigates to another page, it will lock the new page and make the previous one available for editing. If they do not move to a different page, but instead just leave the plan, the page will remain time-locked for 5 min.
Note: The page will unlock after 5 minutes, if you haven’t saved anything or refreshed the page within that time.