When two SIMs are inserted:


You can specify which SIM to use for mobile data, calls, and text messages:
Select options:
Touch Mobile data and select a SIM.
You can also touch & hold 
When two SIMs are inserted, you'll see them referenced in apps like Contacts, Messages, and Phone. To tell them apart more easily, change the default names (SIM1/SIM2) to more meaningful names. You can also change SIM icon color to make them easy to differentiate.