You only have one opening and two good candidates, but one stands out. You hire that one and it is now six months later and you’re forced to admit that you made a mistake. It’s time to cut your losses, start over, and look for a replacement.
But, maybe not, IF you handled the second-choice candidate well.
Did you turn that person down with dignity? Did you keep the door open? Did you say you’d keep their paperwork on file should something open up? If so, you could give that person a call and explore the opportunity you now have for them. The worst answer you could get is that they’re not interested and, even so, you can ask if they know of anyone who might be a good fit.