usually the disconnector spring is what adds the force for the second stage. The disconnector bumps up against the back of the thingy on the hammer that engages the sear and the disconnector. The weight of the first stage is controlled by the hammer spring, the trigger spring, and the angle of engagement between the hammer and the sear.
Of course, there are about 5000 different 2 stage triggers out there now, so your mileage may vary.