Note that the playing pieces from Blokus Duo and Blokus are identical. The differences are as follows: 1. Blokus Duo is 14x14 instead of 20x20 2. Blokus Duo requires players to begin from a square 5x5 from the corner instead of from the corner.
It is easy to play a Blokus Duo game on the classic board, just mark off a 14x14 play area, and away you go! The following images illustrate how to do this:
Marked off playing area
Close-up
Completed game
It's definitely quicker and better playing this way than the two-player variant (two colours for each player) that comes with the full-sized game. Note that the 1x1 playing pieces in the images above just mark the starting squares for each player - you do not actually begin with the 1x1 playing pieces in play, but play them as usual.