Hi Robert,
Unfortunately it is not, and the explanation is quite simple. Compare it to a .txt file, you can open it multiple times (say notepad.exe screen 1, notepad.exe screen 2 and notepad.exe screen 3). If you make edits in screen 1, they are not being reflected in the other screens (they have loaded into memory), but after you save your edits, the edited version is on disk. Notepad.exe screen 2 and 3 still have the loaded version in memory from before the edits and are not being updated. If you do not reload them from disk, a) you will not be beware of the changed disk version and b) you will overwrite the (in the meantime changed) disk version with your edits. Edits from the first screen disappear of course, they are being overwritten.
So to be in control of what changes are being made, it is recommended not to edit in a projectfile with multiple users at the same time, you could (or will) end up with unpredictable results and changes from some of the editors lost.