Was it realistic for Hannah to have enough strength to cut her second forearm after cutting the first so deeply in 13 Reasons Why? King Ecbert did the same in Vikings and I was wondering how realistic it is.
It's entirely realistic (unfortunately)
Sometimes, characters want to end their own lives. It’s tragic, and it’s heartbreaking. And if you’re reading this and considering ending your own life, please don’t.
The issue here is to cut deep enough to hit the artery so that it makes the blood loss so rapid it's, for all intents and purposes, impossible to stop if not caught quickly enough.
What this does not usually do is sever muscles/tendons that infer actual strength.
Assuming the character can cope with the pain and the blood flow isn't sufficent to cause unconciousness, then it's possible to slit both arms (it's probably incorrect to refer to wrists here) before passing out from pain/loss of blood.