How can 2 different Garmin topo 24k maps (on 2 different Garmin sd cards) transfer to a single usb flash drive for use with Basecamp? The two different topos I've tried but only one of the topo maps shows up in Basecamp maps.

How can 2 different Garmin topo 24k maps (on 2 different Garmin sd cards) transfer to a single usb flash drive for use with Basecamp?  I've tried  but only one of the topo maps shows up in Basecamp maps.