Well, two thoughts.
The first is that, even if you can't properly acoustically treat the room, the addition of "soft stuff" like curtains or movers blankets around the place will help deaden the echoes. When in one previous house I had to record in an untreated room, position the mics so there were soft, thick curtains behind the person speaking/singing and a wall of book shelves with very uneven books on the opposite wall (i.e. behind the mic) helped quite a lot.
The second is that, if you can rearrange things, placing your desks to face each other would minimise cross talk between the mics. The AT2020 has strong pick up from the "proper" side and almost none from behind it so if you can be in each other's dead spots it'll sound better.
Third (I know I promised only two but I never plan ahead), you don't mention mic stands but if you can get two floor stands with boom arms rather than desk stands, this will hugely reduce thumps and noises that would be picked up as you hit your keyboard or bump the desk. FYI, for your needs, two of the cheapies that you can always find on eBay would be fine. I wouldn't recommend them for gigging musicians but around the home they should last okay.
That's what I do. I drink and I know things.
-Tyrion Lannister (and Bobbsy)