I would think it would be a single logical drive, but I've never tried it (my drive can do it, but I've neve bought media).
Reason I ask is that a "seamless layer change" was a HUGE deal back in '97 when they figured out how to do it -- apparently it was a nightmare to author and press, and companies like Warner Brothers were burning the midnight oil trying to get it right. (Not a dual-layer disc -- those were common. I mean a dual layer disc where the movie starts on one layer and ends on the other, with at most a stutter when the player switches layers.) If you can do that today by pushing a button in Nero, color me impressed.
RQOTW: "I'll make sure that our future is defined not by the letters ACLU, but by the letters USA." -- Mitt Romney