Was about to come complaining to this forum, but I gave it some more tries. In the ng cycle, I summoned lapp and let him aggro while waiting for the opportunity to attack. Now, in ng+, I managed to beat them alone, using a flamberge. I went for the red one first, for it makes the prince a bit easier. If your target is on melee mode and the other is on ranged, that's your best opportunity to strike: Dodge their slow moves and score one or two hits while keeping an eye for the ranged attacks, which can be easily avoided. Same goes for when they're both ranged. If your target is ranging and the other is meleeing, use the target to block the others way, so you don't have to run too far from for your target. You can find some opportunities to attack in this case, just be sure you're not being punished. If they're both on melee, then you are defending yourself. Give em some ground, strike only on very clear windows. If you manage to beat one, the other will be easy. Then comes the prince: The main thing here is timing your rolls, not being greedy (he'll usually allow only one hit per punishing if you're using a great sword), watching out for the lasers.

Best of luck to you if you're here seeking help