Actually you could run two #12s from a 20 amp CB to a single receptacle.
The parallel conductor rule is for the protection of the paralleled conductors. It allows the OC device to be much higher than the required level for any single conductor. If one follows the rules for parallel conductors, the current will divide equally to all conductors, and the OC device will provide the required protection for the paralleled conductor assembly. If the rules are not followed, then current flow will not be equal in each paralleled conductor, and one may carry much more current than the others, and be overheated, damaging the insulation by heating it beyond its temperature rating.
However, if the OC device is low enough to be sufficient to protect any single conductor, then go ahead string your circuits such that there may be several simultaneous paths from source to load (parallel paths). All conductors are protected from overheating, no damage can occur, and the intent of the NEC is maintained. (not withstanding the appliance or motor disconnect rules).
There is a difference between paralleled conductors and switching arrangements in parallel.
Ring circuit arrangements are a different story. There is a NEC requirement to run all conductors of a circuit together to eliminate inductive loops. That is why we only find ring circuits on overhead spans of utility distribution lines.
