Joining two wires into a single breaker
You can simply pigtail them in the panel - splice those two wires to a short third wire (pigtail) of the same gauge and type, and land the other end of the short wire on the breaker.  

Wire nuts are most commonly used for splices, but they do take some skill to use successfully.  Push in wire connectors are also commonly available, and are much easier to get right on your first attempt.  In either case, you must be very careful to select connectors compatible with your wiring.  In particular, be very careful if there's any aluminum wiring involved.  

There is a persistent myth that the NEC prohibits splices in panels, but there's really no such rule.