Simple 2-player rules for
Use the 4 player board. Each player takes 2 races and colours and plays those pieces separately.
TWILIGHT IMPERIUM 3
Before the actual game begins each player secretly writes down which is his "primary" race. A player automatically wins if his primary race reaches 10 points. If a secondary race reaches 10 points, or the Imperium Rex card is drawn, then whichever player's primary race has more points wins.
The result of this is that the secondary race would likely be used as a spoiler to prevent the opponent from winning, which may add more conflict to the game.