When you’re in an app that requires SIM (like calling or messaging), swipe your finger down from the top of the screen to see the pull-down menu. First press the Quick Settings button (white icon on the right top), then press the icon with the two arrows (data connection) and activate the SIM you prefer.
Both SIMs are active at all times for incoming calls/texts, but 3G capability is only active in one of the two. For outgoing calls/texts, pick the SIM card you prefer before you make a call or send a message using the method above. You can also assign a specific SIM to each of your contacts so that you’ll automatically contact them using the right one.
For internet access, swipe down from the top of the screen to open the pull-down menu. Press the button on the top right, then press data connection. This will enable the internet connection on the SIM card you choose. 3G capability is set by default on SIM 1 (left). To change 3G over to SIM 2, go to System Settings > Wireless & Networks Section > More > Mobile Networks > 3G Service > Enable 3G > Choose the right SIM.