#12 is good for 20 amps...Make both runs 20 amps.
At the breaker box, use a 1 inch dual 20 amp independent leg breaker. That will use the same blade for the unit, guaranteeing the same phase is used.
When dressing the wires in the panel..keep hot, neutral, and ground of each wire in very close proximity to each other, as far up to the breaker screw as possible. You want to make sure the magnetic fields of the other loads cannot get into that loop. If possible, arrange it so that the new breaker is directly next to the neutral and ground strips, so those wires are near the hots till the very last moment. And if possible, put the new breaker as close to the main mreaker as possible. That will reduce the loop pickup from the stereo lines to the other house loads via the buss structure, not to be confused with IR drops.
If possible, keep the heavy load circuits in the panel away from your new ones.
Transient pops from heavy 220 volt loads can be reduced by either tying the two hots or twisting them lightly..but I am not sure if that is allowed by code.
The wire runs to the outlets, if you can, twist each wire independently, I know it makes install a little worse, but you eliminate the loop pickup between each wire, and subsequent circuits.
Do not use the strip and push feature on any 20 amp circuits, it is not allowed by code...the outlets will specifically state this somewhere, should they have that option.
Do not connect grounds or neutrals anywhere, especially at the outlets. That would allow 40 amps to go through a wire without tripping a breaker.
Good luck
Cheers, John