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.

I answered my own question. Different Garmin SD topo's have many files with the same names. When transferring to a usb, options come up to skip files with same names, etc.  Just have to select option to transfer all. Win 10 will just differentiate with a #2 added to a duplicate file.