Using two 14 AWG wires instead of one 12 AWG
Electrically, multiple smaller wires in parallel act as one larger wire.  At DC, it's strictly about conductor crossection area, assuming the same conducting material (usually copper) is used in all the wires.  With AC, multiple separated wires are actually better than a single wire with the same crossection area.

However, if this is for line power, then your question becomes a legal issue, not a electrical one.  I suspect that it's NOT allowed to parallel multiple smaller wires and then assume the whole is rated for the sum of the currents of each of the wires.  I'm no expert on the electrical code, and it varies by jurisdiction anyway.  You have to consult your local electrical code.