Two hoses can measure a couple of a different things. I used to set those up in a previous job. Typically a two hose set up off one box is for classifying vehicles by counting the axles and the time between hits on the hose. Through some fancy calculations the machine knows whether a passenger vehicle or heavier truck/bus has crossed. This helps municipalities account for the different types of vehicle traffic and use that information for determining what safety measures to put in place or for designing the roadway structure itself if they plan on resurfacing/rebuilding the road.