Multiple NameMapping files to Single Project:
The feature of having multiple namemapping will help us to manage the object separately when we automate multiple application in a single project.
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You are operating under the assumption that the NameMapping file is what does the work. That's not the case. The NameMapping file provides the information for an internal engine of TestComplete to identify objects within applications under test. Dividing the NameMapping into multiple files in the same project would require a pretty significant change to the internal engine processing of TestComplete. Not saying it's impossible, just that it's not an easy change.
Meanwhile, as cunderw mentioned to you in your other thread, proper use of your source control tool can mitigate a lot of this. We use TFS and, for a while, were running into similar problems with conflicts and such. We instituted the following steps/policies
1) More frequent, smaller checkins of new changes
2) Prior to any check in "get" the latest to merge down the most recent copy of the NameMapping
The combination of these two items has helped a lot with having multiple people (4-5) checking NameMapping changes into our source control with minimal major conflicts.