I would just hide the top one, but they have exclusive information they can display.
The top one is a status line and the bottom one is a status bar. If there were only one of them, you wouldn't be able to see error messages and information at the same time.
However the actual reason is probably that Vim has them separated, if I were implementing this I'd probably do the same thing out of habit.
By the way, not sure how well it would work for vifm, but in pentadactyl extension and in newer versions of firefox, there is essentially a single line and it's annoying when some temporary information hides something that's always there.