I've just beaten these two on SL20 with +2 weapons, as part of creating a twink build without "cheating" (getting items from another player/character).
I'm not very a particularly skillful player, so you might find my strategy interesting. I summoned both Orbeck and Sirris, and used the Skull Ring to keep them alive for a useful period of time.
My weapon was a raw +2 Dragonslayer's Axe buffed with lightning resin.
It took many tries, but eventually a combination of good RNG and not **** up paid off.
Highlights of a good run: Orbeck's casts Pestilent Mist two or more times, and Sirris remembers to back off after attacking Lothric (while reviving Lorian) to avoid the WOTG blast.
Oh those sweet words: "Mark my words, Ashen One... You remain among the Accursed." Bliss (my wife thought I'd gone mad).