Also good but maybe weird: Alpha/Beta, Red/Black (and if we need 3+, I think Hearts/Spades/Clubs[/Diamonds[/Jokers..]] is fine if you write down the object of the game, so to speak)
Slightly humorous but not best: Hivemind/Drone, Hypnotoad/Zombie, Sauron/Saruman, Don/Capo, Drummer/Dancer
In some situations it's "process A allows writes, process B only allow reads", in which case "writer/reader" maybe?
In other situation it's "process A is in control, but process B will take control if process A fails". In which case "controller/fallback", maybe?
Wikipedia has some suggestions: https://en.wikipedia.org/wiki/Master/slave_(technology)#Term... Also see the HN discussion: https://news.ycombinator.com/item?id=17954823