I put the C and D Region maps on my Oregon 600 and put a waypoint on the border of the 2 maps (for fast viewing the maps on the gps).
I can enable/disable both maps separatly. Enable one of the two maps works fine, but also enable both maps works fine. Around the waypoint it looks then as 1 combined map even map Region D includes a part of map Region C.
Also a route from map D to map C, with both maps enabled, works fine on the gps.
I think Garmin has made a good decision to use the same FID for the maps, with a different vendorID.
(So there is no need to join these maps with GMaptool.)