pipe both, stdout and stderr in the fish shell

That syntax works in fish too. A demo:

$ function cmd1
      echo "this is stdout"
      echo "this is stderr" >&2
  end

$ function cmd2
      rev
  end

$ cmd1 | cmd2
this is stderr
tuodts si siht

$ cmd1 &| cmd2
rredts si siht
tuodts si siht

Docs: https://fishshell.com/docs/current/language.html#redirects