Thank you for all the replies and the link. Actually, I have just finished reading carefully the "externals definition" section of the book from top to the bottom. I think "externals definition" is the answer to our needs. Sorry, I didn't study "Vendor Branch" section.
I am planning to put project/aaa/ in company B's repositoty, which the partner can access. Then in the main tree, hosted in company A's repository, at project directory level, I set the property of externals definition to point to www.companyB.com/aaa_repo
