tap-a-draft carbonating

[Pacific Homebrewer]

I just bought a tap-a-draft system for home tap beer. I understand that I can force carbonate a 6 L PET keg (in about 5-7 days) or use priming sugar to carbonate and the CO2 to simply dispense my finished beer.
My question is: Does anyone know of a cap or gizmo that will allow me to force carbonate my next 6L keg about a week before the first one's empty? Or do I have to buy 2 tap units?
thanks

[alan_marks]

DO NOT FORCE CARB WITH A TAP-A-DRAFT!!!!!

I can send you pictures where the bottom blew out of not one, not two but THREE of their bottles!
What I have done is priming the beer using half the usual amout to reach 2.5 volumes (or so), condition for 10 days then chill before conecting to the tapper. One of the warnings I have received from SBG was to only connect the CO2 cartriges when the bottle is in a horizontal position. Kinda precludes force carbonation.

hope this helps,
Alan