I put the game down for awhile and now forgot how to stealth kill on the ps4? im at the section with a bunch of 2 headed guys and cant remember how to use the knife for a stealth kill! argh lol..

You crouch down with R2 (I think?) and sneak up on them from behind and press the square prompt whence shown, or can alternatively shoot a flash bolt so you can then stealth kill them from the front. Though I don't believe you can actually stealth kill the AlterEgos.

Pretty sure there's a button prompt for that when you sneak behind an enemy. I'm thinking R2/X from the top of my head.

...not sure how you stab a guy in the head, when there's two heads, though? Does Sebastian just stab really fast? Also, I'm still confused why they even added those stealth mechanics into the game, when it relies on you shooting your enemies to bits in so many cases.

AlterEgos, the two headed dudes you're fighting, can't be stealth killed. The only way to easily take them out is to take out it's main head.

ah I was starting to wonder. thx for the info guys.