Early 1900's sounds like a interesting choice, but I may be having a hard time imagining it because it'd probably look more like webs of alliances.  Or maybe that's just my biases caused by thinking of WW1's prelude and the concept of great powers that permeated that time period. (early 20th century would work nice for a small group NRP however)  I can't help but imagine france vs Britain when thinking of a Victorian Cold War which I lack interest in since I like Cold Wars where the division is based around ideological divides that transcend groups of individuals.  Gods are individuals, admittably, but gods to a strong extent represent ideals more than people.

I am naturally apt to go with a space opera set-up in general due to the sheer theatrics and situations you get in a galactic sandbox with two big kids throwing sand at each other.  The scale would be rediculous, but that's how it is in every space opera anyways.  Due to there only being two players there's less need for balance outside of us as well since the super powers will have all the good toys anyways.  Almost all the civs/species would be basically NPCs.