The latest version of Firefox OS lets you use two SIM cards in one device - it's like having two phones in one. This article shows you how to manage your dual SIM card settings.
You can view the network status for each SIM card at the top of your home screen:
The SIM manager lets you specify which SIM to use for outgoing calls and messages:
You can customize call settings for each SIM card through the Call Settings menu:
Close the app when you've made your selections. Your changes will be saved automatically.