Curumo (Saruman) is not known, AFAIK, to have forgotten his identity and his mission from the Valar. Even ignoring his commitment to the Valar - he knows that Mairon (Sauron)'s master, Melkor, is gone; and that he won't be coming back; and that the most Mairon can do is basically provoke the Valar into slightly more intervention. Hell, even if Mairon has a stable "victory" - what's the use in that for Curumo? It's not as though Mairon can mount a challenge of the Valar.
So why would Curumo even consider joining forces with Mairon? What did he expect to achieve? If he thought the opposition to him by the Istari, Elves and Men was going to fail - shouldn't he have just gone back to Valinor, or barring that, taken up a chair at the Gray Havens somewhere and watched it all crash and burn?