Ellinas is correct. It won't work to join both countries' maps into one img file because it'll be over 4GiB [unless you only have a portion of one or both countries to keep the file size under 4GiB], and as stated there's no practical reason for wanting access to both countries' maps at the same time, although for a period Garmin offered 'trans-continental' models with both EU & NA mapping, that's some time back but not anymore [they came to their senses i reckon ]. However with recent firmware your 260 can have two maps on an SD card in Garmin folder, named gmapsupp.img and gmapsup1.img respectively. You'll need a minimum 8GB card for full EU & full NA maps. See: [Only registered and activated users can see links. ] and [Only registered and activated users can see links. ]. However the heavy map data will probably slow the boot time because of 'loading maps' by the under-powered processor in the unit. Therefore it makes good practical sense to just use 2 SD cards as advised in above post.

NB: nuvi units can't recognise NTFS format, only FAT16 & FAT32 in media cards. exFAT isn't seen either.