So I'm guessing that they DON'T know they are archenemies at this point?
Great idea for a series, though for sp,e reason I'm reminded of the Animated Tick episode features Thrakkorzog, the terribly polite invader from Dimension 14B, in "The Tick versus the Uncommon Cold". He lives in the same apartment block as Arthur and the Tick.
That episode also has the Tick defeating the villain's principle lackey my means of a method so disgusting that I cannot even think about the scene involved without genuinely wanting to be very, very sick.