As that odd person who has rocked up to a barbecue we've been at with their bottle of organic wine and wearing a 'meat is murder' T-shirt, our society still hasn't resolved the issues of eating non-sentient creatures to a perfect consensus. We have pockets of people who complain bitterly about GM foods for instance, but don't know that these foods have literally averted famine in the past and are still our best hope for doing so into the future.
The bottom line is that when there is plenty, you can be as selective as you like about what or who you eat. When there isn't, Maslow's Hierarchy of Needs kicks in.
Changing a meat diet to fish for instance has already been done in the European past; many crusade knights ate a diet of fish as their exclusive protein on grounds that it was considered impious to eat meat. But in order to do so, there had to be a plentiful supply of fish to support those knights. Fishing industries picked up to support that need, and pasturing of sheep and cattle waned except for wool and dairy production. Of course, many of the peasantry still ate meat so it wasn't killed off completely, but the important thing is that there was a fundamental change in the balance of food production industries.
For people who need to eat meat exclusively, and if fish isn't present, the best fictional analogue to your question is that of vampires. If we think of vampires as the carnivores in your model, and normal humans as the herbivores, it's almost an identical model and many of the issues you raise are covered in various fictional works.
Want to swap out 'meat' for an alternative source? Look at True Blood. Want to look at Vampires as trying to live in harmony with humans and other races? Try Twilight, although I admit that it's a problematic example. Want to see the possibility of a hidden ruling class of vampires? You've always got the Underworld Franchise.
The list goes on.
In all cases, you'll find exceptions to the rule because not everyone wants the alternative in both food and culture that the protagonists represent. Someone wants true human blood, or open control of humans and/or vampires, etc. Your society will still have those kinds of power plays.
Ultimately, humans see themselves as the only truly intelligent species on Earth and we abhor cannibalism because it's bad enough thinking of eating a creature at all, let alone eating a creature that can think. But, carnivores are unlikely to see their world in quite the same way, even if they become intelligent and even enlightened.
The herbivores are still peasants; they're still food.
I'd argue that the carnivores are highly unlikely to allow the herbivores to be come intelligent, let alone privileged in any society. Just like we've selectively bred dogs to be less intelligent (and more reliant on us) than wolves, like we don't breed sheep or cattle for their cognitive abilities, intelligent carnivores are more likely to 'farm' herbivores, not uplift them to a similar status to themselves.
What you describe as cannibalism is really just cross-species predation, in a world where the prey is intelligent. Intelligent carnivores are actually going to breed that out of the herbivores because it's a complication to their food supply they don't need or want.
So; as I see it, your society will function in one of two ways;
1) Your carnivores switch to fish exclusively and then embrace their former prey as a member of their society, making it both culturally and emotionally unpleasant to eat their fellow citizens, or
2) Your carnivores continue to eat the herbivores, but farm them rather than embrace them, breeding out the intelligence wherever possible to make their farming practices easier.
If you go with option 2, your mixed species society is impossible. With option 1, there are some really interesting explorations to make around the moral and emotional decisions your carnivores may one day have to make if the fish run out. But, there may also be some members of your society who simply don't believe in a picotarian diet and end up becoming a form of serial killer in your society. There is of course also the issue of whether or not a former prey can ever truly be seen as equal to your carnivores.
For inspiration on how option 1 may look in reality, there are plenty of vampirical and post-apocalyptic works of fiction that explore these kinds of moral dilemmas in a world where you just have to eat.