No one knows for sure.
I'd guess it will be like Eevee is right now - there may be a one time "forced" evolution with a naming convention, and otherwise it'll be random between the two.
I'd hope it puts two separate evolution options and you can always choose. That's wishful thinking.
I've also read the idea that pokestops may provide an item needed to trigger the new evolution. Not sure how practical that would be but it could work as well.