That's missing a `nub` or something to get the unique keys. You can also combine the maps first and the results will come out sorted and unique already.... since we're golfing:

    go :: (Show k, Ord k) => Map k v -> Map k v -> String
    go m1 m2 = orNone . intercalate ", " . fmap show . keys $ m1 <> m2
      where orNone "" = "<none>"
            orNone x  = x