The oldest post becomes the first post. When threads are merged posts are put in according to the time they were posted. That's why mods are so careful with merging threads, as by merging two threads that overlap, you'll suddenly have a disruptive conversaion in the thread where both threads overlapped. This is however not he case with those threads, so when they are merged it will be a smooth transition.
As for the updates, why don't you just update the OP of one thread and c/p over in the other? That way both will be up to date easily.