Amazon must have taken down the trailer, but I did see it the other day. Unfortunately I missed Zaphod having two heads (I was at work so maybe I wasn't paying as much attention) but they did have his third arm. The problem is that the HoG is in the wrong shape (wasn't it shaped like a shoe?) and other than the beginning parts (goodbye world) I don't remember any of the events in the trailer happening in the book. I guess I just need to read them again.
They aren't side by side. One is on top of the other. At one point in the trailer the single head you usually see tlitls back, and the second one rises up out of his collar.
I'm guessing they did it that way so they didn't have to worry about showing two heads all the time. (In the old BBC TV show, the second head mostly just hung there looking lifeless. This movie won't have that issue. )
-S'Cipio