I'm just going to throw my two cents in but please don't think I am thinking it is a bad idea because I don't. My only concern for you is that traditionaly a drop panel is heavy enough to have some weight to it which results in a loud bang or thud. That noise is the initial scare and an actor is an added bonus effect; put the two of them together and you have a great scare.
The only down side that I see is that one panel drops and your initial scare may send some running and your real scare hasn't even gone off yet. One way to help your idea is to not weight the first panel down and to cushion the drop with something thus allowing the second drop panel to be the effective one.
My vision is that you have this scare down a hallway (as previously mentioned by another) dropping the first panel while the "target" is still a little away and approaching it. When they come closer than drop the second panel (heavily weighted in my opinion for a better scare).
Just my thoughts.... but please let me know how it works either way you do it. I'm anxious to hear.