Ornstein and Smough.
Something happens to my brain when facing two independent bosses. Like I just can't cope with it, and I inevitably end up sucking.
I can do Midir fine (great fight), Gael, Twin Princes, Nameless King... but I choke on simple Ornstein and Smough.
Luckily, it's really rare in Souls to have fights like that. Most multi-enemy boss fights are simply duplicates (gargoyles) or have a gimmick (Friede) that makes it easy to adapt to there being more than one enemy.
The headless ape duo fight in Sekiro almost broke me too. It only took a few tries (their health and posture is comically low), but I still screwed up royally (at one point, I was just pushing buttons—I couldn't even remember how to fight for a few seconds).