Why does the drive thru operator change back and forth during my order?

I just had two people take my order in a drive thru. They alternated every question. First person welcomed me and asked what I wanted. The next person asked what size and drink I wanted. The first person came back on and asked if I wanted any cheese sauce then the second person came back on and told me the price. When I got up to the window the 2nd person was the one working the register. This happens from time to time at different places when I get lunch but I can't figure out why it is done.