It seems to me the two zipper system has the following advantages:
1) You can localize your opening point. In your example if your want to access something in a top corner one zipper might take two or three corners of travel to make (since it only closes at the ends). Two zippers might be immediate depending on the location you chose. Travel time is typically less with two zippers (and this does little difference for small containers).
2) Two zipper system tends to adapt better for both left handed and right ended persons and the position of the person relative to the container. It is more flexible altogether.
3) In some cases distributing by two zippers the tension applied might be very advantageous (usually school bags or other such where it is not unlikely to be filled with more stuff than its supposed to).