As pointed out, the second date is the date by which you must depart on that particular visit. It does not entitle you to re-enter after departing (nor would having a multiple-entry tourist visa either) as you must show you meet the requirements of the VWP on every entry -- this is true even if you take a day trip to Canada or Mexico on one of your visits.

In reality, as long as you have a good explanation for why you are visiting and can prove you are not seeking to emigrate to the U.S., you will be fine. For instance, if you have a job in the U.K. and you are on leave while visiting the U.S., you shouldn't have any problems.