2; an accident and a slip
They're only children, really, but the war's taken so, so much out of them both. Harry isn't the same, either, but that was to be expected – the boy was born to be a martyr. Hermione was born to be a genius, never caring about much other than her own intellectual pursuits, and Ron was born to be a sidekick, always hindered by his own tragic shortcomings. They were born to be two, not one. Two people, after all, still have a chance at life, if they can manage to abandon each other (and they can). One person will surely die. One person has no hope of anything more than a young death and a forgotten eulogy, and a headstone in a darkened graveyard at the edge of town that no one will ever visit. One person has no hope.
But Ron and Hermione would rather die together than die alone, and so they vow to stick together, no matter what. Even though it's a mistake.