"Theoretically" - it depends on what the breaker size is. If it is 30A (which the wires would support) they have to be the same length (or else the current is not split evenly between the wires). On a 15A breaker "theoretically" it doesn't matter. The OP seems to be asking the first question.
For paralleling under the NEC, not only do the lengths have to be equal, you have to match conductor material, conductor area, insulation and termination.
As Doug said, the NEC allows paralleling for wires over 1/0. Why would you want to parallel small wires. You are increasing the probability of a problem, including what someone might do in the future. And the "right" size wire is about always easier to use.
As I believe someone said, wires have to (generally) be protected at the source for the wire ampacity. A #14 (or 2-#14s) connected to a 30A breaker is a violation.
I have read that ring circuits in the UK were used in the rebuilding after WW2 because they used less scarce copper. If the ring circuit was
30A, the wire would be lighter than a 30A rating, but I believe it was significantly higher than 15A. Since each side of the ring would seldom be equal length, the current does not divide equally in each direction. You also don't want "sockets" near the ends of the ring.