I have two compositions, second is a copy of the first. On the second's layers i use other footages with other start points, while the keyframes of the layers should stay there (related to the layer). I have replaced the the second's footages by alt-drag&drop. But i can not set other start points.
So how can i shift the footage behind the layer?
What stops you from just moving around the keyframes? Not quite sure what you are doing, but it seems to me you are overthinking this terribly instead of just biting your way through...