Actually, I'm not sure what it does, because I noticed it will measure also with only one scanner, and I did a loop with no problem.
Anyway, I was just told in another thread to add "Display custom HUD" in "edit session rules", and THAT is the way to go. It adds an odometer to your HUD, so you can just drive the section or the whole route and you'll know the distance, cool! Has a few more little neat features as well.