The file locking feature of the professional version tries to solve the problem automatically for office files (by stopping to users from editing the same file). Other files can be locked manually by the user. With the explorer extension enabled other users can see that the file is locked (and I think they can only open it read only).
Locking is the only thing I could think of. Other than that I could only think of editing the file within the browser (using Collabora Online), but I’m not sure if it is already supported that two users edit the same file (and it is limited to some file types as well).