For more information on your device, select it on the Support main page.
Basically, you can use a dual SIM phone just like a phone with only one SIM card. When you send messages or connect to the internet, Nokia Suite uses the SIM card that is in the internal SIM card holder of the phone. This card goes by the name of SIM1, unless you have renamed it.
To send a message or create an internet connection with the other SIM card, swap the SIM cards in your phone.