I don't think it was mentioned above, but you'll want to set the kettle up for an indirect cook, not for grilling, and I'm not sure if there will be room for two briskets. You may be able to rig up some way to stand them up on on edge to give you more room and then rotate them part way through the cook.
There is a good diagram or picture around here that shows setting up a kettle for an indirect cook. Let me see if I can find it. BRB
OK... I'm back...
Our resident kettle smoking expert (at least that's what he calls himself
) shows various set up for smoking on a weber kettle.