2 different circuits in same double gang box

I am adding another switched lighting circuit to an existing box, which i will make a double gang.
I believe that is not a code problem, but should I keep the grounds in the box separate for each circuit, or should i tie all the bare ground wires for the 2 circuits together?

 

Tie all the grounds together.
Keep the white wires separate from the 2 circuits.

 

With residential electric you always tie the ground wires together in a junction box, regardless of which circuit they came from. It is only hot and neutral wires that you keep separated by circuit.

To comment further, some people do not recommend what you have done and consider it unsafe. They frown on this because there is the possibility that someone might turn off a breaker to one circuit but leave the other breaker on when accessing one of the switches in the box, thus leaving open the possibility of electrocution. My suggestion is to make sure that you mark the breakers so that someone will at least be warned that the breakers power devices through a common junction box.

 

This is done all the time and is code-compliant. One particular place that comes to mind is next to a kitchen sink. A double gang box holds a switch for the light over the sink and one of the small appliance receptacles.

As was previously mentioned, twist all your bare or green grounds together and secure them with a crimp-cap or wirenut.