Is there an Eldritch Horror variant for playing against two ancients at once?
No playtesting yet, but I was thinking about such variant.
Some initial thoughts:  


I would sum up the doom of both AOs. If this total drops to 0, both AO's awaken.
The two respective mythos decks are shuffled: First stage of 1st AO with 1st stage of 2nd AO, and so on.
Each AO has one active mystery. "Advance the mystery" effects allow you to advance the either of these, but not both at the same time.
When having a Research encounter, you may choose from which AO's deck you'll draw.


The biggest dilemma for me is how to treat Cultists.