Can I feed two circuits from one breaker?
Yes, it's fine to combine circuits onto one breaker, at which point they become one circuit.  Provided, of course, that there is not a Code prohibition on that due to certain particular circuits needing to be dedicated, i.e. You can't add anything to a furnace circuit, and cannot add non-bathroom loads to a bathroom receptacle circuit. 

Mechanically, you must use breakers listed for your panel by UL.  Your breakers might allow 2 wires on a terminal, if not just pigtail them. Wire nuts are allowed in panels.