Woohoo! My two favorite legendary characters are finally on-screen together! :o)
Although they don't interact almost at all, they do share the screen in a couple of scenes during the first half of Once Upon a Time's fifth season.
Although King Arthur is integral to this half season (since part of it takes place in Camelot and he is later transported to Storybrooke), poor Robin Hood doesn't have much to do. Robin is mostly just hanging around Storybrooke waiting for Zelena to give birth to their baby (discovery from the end of season 4), being saved by Regina, and helping Hook break into Emma's house to find out what's behind a secret door. He doesn't have much to do in Camelot either, unless you count almost getting killed and being extra muscle/arrows from time to time.
Sadly not much in the hero line... nor any real interactions with Arthur (other than an encounter at the Storybrooke jail in episode 5 and Robin's participation in the group assault to steal Excalibur from Arthur in episode 7). But it did give me a kick to see them on the screen together! :o)