Are there any playable Twilight Imperium 2 player variants?
There is no official 2-player variant; the game just wasn't designed for it.

But as always, the internet provides.  Various fans have proposed two-player rule sets.

The Roland Wood variant is probably the most commonly used, building on earlier suggestions by other players.  

(There was another popular version in the "Shattered Expansion", but I can no longer find a live link.)