One of two ways:
1) Right click on the project and select "Add -> Existing Item" and navigate to the NameMapping.tcNM from Project A and add it to Project B. The benefit of doing it this way is that, anytime you update object identification in either project, it will immediately update it in the other project. Requires Project B to not have a pre-existing NameMapping node... can be removed/deleted from the project if you desire to go this route.
2) Right click on the NameMapping node of Project B and select "Merge" then navigate to the NameMapping file from Project A and complete the merge. The benefit of doing it this way is that you can isolate the NameMapping for the two projects so that changes in one don't affect the other.
On a side note:
Please find and view a TestComplete 101 webinar... you can attend one live or watch the recording of one. Either way, this will give you the basics of TestComplete. NameMapping is one of the core pieces of TestComplete and is the default behavior for object identification. Everything dealing with onscreen components references NameMapping. SO, get an understanding of NameMapping and you'll have a good portion of TestComplete known... and the webinars cover a good bit of this.