Since you only use it when you've missed, it doesn't really matter.
You know your first die was a failure, the only question is whether your second die will be a success. Even if its a worse failure than the first die, it doesn't matter which one you select because a failure is a failure no matter how bad.
There's no requirement that you use elven accuracy when you miss; one of the more ingenious uses of the ability is to drop an area of effect spell on a bunch of enemies surrounding an ally, and then reroll against the ally if you hit.