Devourer assist stacks

So a friend and I went into a custom game because we were debating whether or not getting an assist on a large monster will give you devourer stacks if two people on the same team have it. assisting on blue buff gives stacks, but assisting on dragon does not. Perhaps this is intended, in which case carry on. however, Riot employees have stated several times that they're not opposed to taking smite into lanes, which would imply that if yi and vayne both have devourer and they take drag together you'd want them to both get the 5 stacks they earned. So is this intended, or is it a bug?