2 circuits in a double gang box
No problem.   Two circuits can share a box, and even serve different switches or outlets in the same box. Be careful to keep hots and neutrals separate. 

For instance a household member really likes running the hair dryer and curler at the same time.  Nothing stops you from running dual 12/2 to the bathroom receptacle, splitting hot and neutral tabs, and serving each socket from a dedicated GFCI breaker. 

You could even do that with different main services, e.g. The above but with one side served with mains power, the other from a separately derived PV or generator system.  However you would need to distinguish each system with different wire markings or colors.  This is why gray is an allowed neutral color.