With double game you start two new games against the same opponent. You play White in one game and Black in the other. The choosen handicap and komi will be used for both games. In the waiting room the number of handicap stones is shown on the info page for the game offer.
 
To start a double game as invitation or as new game in the waiting-room you have to choose Double from the select-box right to My color with a choice for Manual setting.