Loads of good info online about introducing dogs to each other. Forget that they used to know each other, and deal with them as you would two dogs who don’t know each other.
http://m.petmd.com/dog/training/evr_introducing_a_new_dog_to_a_resident_dog
Personally, I’d starting a neutral place, outside with some space so the dogs don’t feel hemmed in. See how they get on, and it’ll give you need idea how they’ll be together.
However, from experience a new dog can take up to 3 months before they show their true character in a new home. During that initial period you may have a far easier time than later on. So just play it by ear, and keep an eye on both dogs to step it if anything happens.