Bob and BITS stated it how I've always been taught. We still have a couple of grade school gyms around here that use the double line. I believe the idea is that the players must travel the "full size" backcourt to avoid the 10-sec. violation by getting past the second line, then once they're in the front court, the first line becomes the "over-and-back" line so the players have a "full size" front court. The two areas are just overlapped in between the two lines.
Good luck sorting who's front court and backcourt when there are 5 turnovers in that overlapping area...
