The best caddie programs are undoubtedly where caddies are mandatory and paid the same rate per bag regardless if they double bag or not. And the caddies that are given the luxury of double bagging to make more money are generally the best caddies at the club, hence the seniority and why they're allowed to carry two bags at once.
Courses that, in my experience, had mandatory caddies were Cypress, Shinnecock, NGLA, and Friars Head. I also had an excellent caddie at Riviera that double bagged although I'm not sure caddies are mandatory there.
Anyways, maybe it's something to be said that the courses with the best caddie programs are also the best golf courses in the world. Likewise, the caddies at these courses are about as good as they get.
Why not make caddies mandatory at Streamsong? If they want to build a walking culture and give the player a full experience this would be a great idea. This would also encourage better caddies to flock to the courses because they would be guaranteed to make money.