Of course!
Orcus becomes the new CE power of undeath, supplementing Nerull and Vecna. (Or you can keep Vecna. Orcus can command him around later). Grummush, after killing the goblin, elf and most other monster pantheons, settles for NE and becomes not only the god of Orcs, but most gobliniods and ogres too.
Orcus and Grummush settle on a pact, Orcus gets 1/3 of his worshipers, and Orcus ensures a 1,000 year long armistice which includes sending out demons and undead to help Grummush in his next fight, Pelor. (Orcus figures he can't take Pelor YET but hey in a long fight, anything possible.)