Combining Two Lines Into Single Circuit
A breaker box is a perfectly legitimate junction box.
Assuming (since you mentioned moving the cable to the breaker) that you have adequate slack in the cables to move them to the breaker box:
So (if the breaker is not listed for two wires - varies with many details, I know the SquareD QO ones I have are, for copper wires of limited gauge range) just put a pigtail on the breaker and wirenut it to the two hot wires from the two cables you want to relocate, and either do the same from the neutral bar (Only ONE wire per hole on the Neutral bar, though) and wirenut to the neutrals on the cables you are relocating, or use two holes on the neutral bar if you have room to do each of the neutrals you are relocating.
Likewise, connect the grounds to the ground bar (which might or might not be the same as the neutral bar, depending if it's a main panel or a sub-panel) and which might be listed for more than one ground wire per hole (check the box label for that information, and the required torque value for the ground and neutral bar screws. Breaker torque value my also be noted, or you may have to look on the breaker itself for that number.)
Use a torque screwdriver (or low-value inch-pound torque wrench) to make the connections properly.