No, the multiplication rule applies whether the mutations occur sequentially or simultaneously.
This is only pertinent from one perspective (the wrong perspective when it comes to evolution).
For example, what would you say if I told you that I could, right here right now, guarantee doing something that has a one in trillions upon trillions of a chance of occurring, and that I can prove it with trivial math ?
Could there be any greater conceit, than for someone to believe that the universe has to be simple enough for them to be able to understand it ?