bcnewe2's suggestion to keep the two separate, and give each a share of the sheep , would be the safest way to go. If the old llama is bonded to "his" sheep, adding the new llama with the sheep present may set up an even more aggressive
attack, with the added thrill of the sheep milling around. Even if there aren't major injuries, and you decided to leave them all together to work things out; you may find that the old llama will bully the newcomer and chase him away from the flock, the feed, the hay, the water, and he'll do so poorly you'll have to pull him out i n the end anyway.
The flip side of it is, if they do get friendly; instead of joining up to protect your flock, they may just as well join up to sunbathe in the dirt by the gate, while your flock wanders off alone. Instead of double the protection, you get a llama group and a sheep group.
Hope it all works out for you!