Multiboxer V2 is about as good as it gets.

With your specific problem, you still have to open dialogue with the NPC on both characters. But after that multiboxer passes your dialogue choices to the other character for you. To my knowledge this is the best solution to the problem.

For shareable quests, you can just pick a bunch of them up on your main and multiboxer has an option you can turn on for the slave to auto-accept all shared quests from users on your friends list or something like that, so you can just go down the list and share them all without ever switching screens (or using an accept macro).