When working on a piece of content as an editor, the content will automatically be locked for others. Otherwise you'll overwrite each others work. Therefore it is not possible for two editors to work on one piece of content at the same time.
In case of a duo-project, one person has to unlock the content before the other can start working. You can also break the lock from someone else. However: make sure to never break a lock without checking whether the other editor is working on the content.
Tip: always commit your work and unlock the content when you're done.
You unlock a module by clicking on the orange 'Unlock' button.