In GalCiv2 terms, it'd be the morally righteous do-gooder Altarians allying with the xenophobic nd genocidal Korath.
But that would only happen if a third party came a long that was hated by the Altarians and Korath even more than they hate each other, and only if the Altarians and Korath understood that their only chance of victory against this abomination is to join forces. Once that's over (assuming they win) they'd be back at each others' throats - and there would probably be a lot of backstabbing and deception between them even during the conflict.
I'd love to see this happen. For even most hated enemies to temporarily work together against something they both think is even worse, rather than choosing certain destruction by going it alone. Like every other aspect of AI, though, I'd like there to be some randomness, some unpredictability - so even if all the conditions are there, this wouldn't always happen. And it might happen on occasion even when you don't quite expect it to.