volant: I think it is impossible to do now and it would be a nice feature. I see so many times someone posting something like 6 waiting games with each color in some unbalanced game, and some time later there are still 5 black games available but only 1 white game - which mean that the contenders take the white challenges but ignore the black ones.
To get around that, you can offer two-games match, but those are then counted (statistics- and rating-wise) as only one game.