Huawei Mate 8 - Manual (Page 30)

First use and initial setup
23
Dual SIM settings
Switch freely between two SIM cards. Manage your work and private life on one device.
This feature is only available on dual SIM phones.
Open Settings and touch Dual card management. You can configure both SIM cards
as desired.
Enable or disable a SIM card
Select 3G/4G SIM
Select default SIM for data services