As someone who has collaborated extensively (according to Jaysen), may I point out that you have that essentially already. When the project is opened, a “User-lock” file is created. If anyone attempts to open it on another computer, an alert comes up saying it is already open and would you like to open a copy — madness lies that way, as your copy and the other copy would differ — so simply click “Cancel” and wait till it has been closed on the other computer.
See 1 Master file, two writers collaborating - any ideas?
Mr X