In a frontal impact, properly restrained, the steering-wheel airbag should helpt, at most, your head neck by preventing severe whiplash and trauma to the head from impacting the wheel or other car-part.
Your chest should never usefully impact the bag, as the belt and seat should have restrained it. Your arms are actually pushed away by it, and leg protection is minimal
There are other airbags, and other impacts, with different effects.
"two impacts" is an oversimplification... there are multiple impacts drawn out over an extended time. The entire time your body is decelerating (which is far from uniform across your body) Is the time where injury occurs.
In a perfect world, your body would not move relative to the car and would be restrained across as much of it's surface as possible: with emphasis on structural supports (bones). For reference, look at the restraint system in a race-car. Take note too that the helmet is restrained to prevent the head from tossing around too much.
