Hello
Go to Settings==>SIM management: To select the desired SIM card for voice call, touch voice call under default settings and select either SIM 1, 2 or always ask. Do the same on massaging.
Data connections: Touch Data connection and select the SIM card with data internet bundles.
Please note: Only one SIM card can run the 3G service (Settings==>Wireless & Networks [more]==>Mobile networks==>3G service==>Enable 3G and select a desired SIM card) and for a batter or faster internet browsing make sure to select the same SIM card under data connection