I have this layout built many years ago on two 36" doors. It has been all over Arizona to various shows, but is now permanently at our local mall display. The two sections are bolted together with a nut and bolt and wire plugs underneath connect the wiring. The first photo shows it with the original O gauge track and the second since I changed the track to Fastrack. You can see the seam between the two doors in the lower left on the second photo.
I got the idea from a friend in Tucson who built his entire HO layout on hollow core doors.