There was a huuuuuge affiliate uproar years ago which caused most programs to honor the last referrer only.
Cookie stuffing by some very large affiliates of the day made the market almost non existant for the rest of us.
Affiliates fought long and hard to get the industry to honor the last referral only. The only allowed exception was cross promotions by the casinos themselves, and systems that left the tracker with the affiliate who sent the player to that particular group in the first place, even if the player went from casino to casino within the group.
You are right Simmo, a lot of affiliates will vigorously resist that type of accounting. It encourages blackhats and cookie spammers of all persuasions. It is bad for the industry.