Can I wire nut two black wires together and hook 1 to a breaker?

Not according to the electrical code. It requires each separate circuit to be fed individually from its own breaker. If allowed it would reduce the total capacity of each circuit from one amp per outlet to a half amp per outlet if the circuit was fully loaded.The work around in situations like this is to use a tandem breaker which gives you the capacity of two complete circuits and only using one breaker slot in the distribution panel board.