Each of the .img must have a unique number. Otherwise they will be in conflict in the gps.
The maps will work in basecamp, but not in the gps.
It is possible to change the ID of each of the 366 .img of one of the maps. GPSMapedit, can do it. Very long.
Before, you can use the Javawa GMTK tool to check the conflicts of your two maps in Basecamp.

Perhaps there is a better tool to change the ID of img?