Replacing receptacles
Yes, the Code prevents you from doing this. On the surface you'd think you can pigtail the #8 wire to #12 (because #8 won't fit in 20 Amp outlets) and connect the #12 to 20 Amp outlets. But the 40 Amp breaker has to be changed out to a 20 AMP breaker because the breaker can't exceed the rating of the wire/outlets attached to it so you'd be limited to using one appliance at a time.The wire might have to be pigtailed into the breaker if it doesn't fit. If these are kitchen appliances on counter tops then the outlets would need to be GFCI protected. If the existing wiring is in conduit, you might think about running two 20 Amp circuits.