The phrase "dual nano" refers to something that is made up of two extremely tiny components or features, typically at a nano-scale size. It implies a combination of two very small elements. Full definition
HTC Desire 10 Pro comes with dual nano SIM slots sporting a 5.5 inch IPS screen protected with Gorilla glass and supporting 1920 x 1080 pixels resolution. (gogi.in)
The Desire 12 will reportedly support Wi - Fi 802.11 a / b / g / n and come with dual nano SIM support, as well as a 2,730 mAh battery for keeping the device powered on. (androidheadlines.com)
The device supports dual nano SIM with Dual standby so it can be used for travel fairly easily if you need a device that can switch back and forth between networks from different regions. (androidheadlines.com)