What we did was allow everyone to do their own mapping but not check it in. Then every week or so, (we rotatethis amongst the team because it's a rotten rotten job lol) one person takes everyone's name mapping and merges them one at a time into an empty project and fixes all the conflicts. Then each person gets a copy of the merged mapping and tries their tests and if it works for everyone, it gets checked in.
I understand your intent but please allow me to express my deepest recommendation of NOT allowing multiple people to make changes to the tcnm file.
I recommend for you to work the way TestComplete would like you to work and not go against the grain.
I hope in the future that Testcomplete will implement a different way of using Name Mapping, maybe even a database like other products do, but until then, life is too short to have multiple people make changes to the tcnm file.