What was seemed to work so far is killing fatty and rolling under Ornstein and just going nuts on his legs.
But unfortunatately, I had no estus and he did the electric butt slam which killed my already low hp. I got him to at least 25%.
The problem is not when they're as one, it's as when they're two.
Summon Solaire. Yeah, it increases the HP on both, but he's the perfect distraction for the first part of the fight. Ornstein should be playing patty cake with Solaire while you drop Smough like a bucket of lard.
"I'd rather betray the world than let the world betray me." -Cao Cao
This fight was the most fun for me so far. Super powered Ornstein is tough. I was down to 1 estus flask. I summoned to guys to help me but they were killed quickly, so I was by myself essentially. Roll and keep you shield up..you should know the drill. Get between Ornsteins legs and hack away. Lots of dodging.