What we have here is a cross-licensing/licencing agreement.
You can find some sample clauses at
https://www.lawinsider.com/clause/trademark-cross-licensePS to add some explanation why assignment isn't a good choice at all:
"In Canada, the Canadian Trade-marks Act allows for transfer of a trademark with or without the goodwill attached to it. In the United States, goodwill must be transferred along with the trademark or that the assignment is invalid (15 U.S.C. ss. 1060). Assignment of a trademark in the U.S. without goodwill or business attached to it leaves nothing for the mark to signify and it loses its inherent function and is abandoned."
http://www.ludlowlaw.ca/DGA publications/Character and Merch...Choose something else, IMO.
Basically, I've already given you your answer, Gavin. I think this isn't about a system to keep track of some "brand score"; it's supposed to ensure that the goodwill remains where it is.
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