Can multiple outlets be directly attached to a single breaker?
a) Two NEMA-5-20 outlets, each has a run of #12 wire, that runs all the way to a single breaker, which then clamps 2 black #12 wires.


It depends on the specifications of the breaker. If it allows 2 wires then OK. If not, just connect the two wires to a third wire (pigtail) and connect that wire to the breaker.


  b) Two NEMA-14-50 outlets, each has a run of #6 wire, that runs all the way to the breaker which clamps on two red leads and two black leads.


No. See Wire two 14-50 outlets to 50 amp breaker