The Mate 8 is one of a few devices that can use two SIM cards at once. This is a common feature outside of the US. You could use it if you have a SIM for one country and a SIM for a different country, or maybe you and a friend want to go minimal and share a device.

The Mate 8 has one SIM tray that can hold both SIM cards. First, use the SIM removal tool or a paperclip to push in the small hole on the left side of the device.



The SIM card tray will pop out a bit so you can grab it with your fingers and pull it out all the way. This tray can fit two nano SIM cards in the orientation shown below.



Use "Slot1" if you only have one SIM card in the phone. The Mate 8 will do all the hard work for you. Any call or message will be labeled by what SIM card it was sent to.