Will two 14Ga. wires handle the same load as one 10Ga. wire in a 12V DC application?
While this may seem OK in theory, it is not a good idea in practice and here is why it could be unsafe:

You are probably assuming that the current will split equally between the two wires, which under ideal conditions it might (or at least be very close). In the real world however corrosion is often a problem and where the wire enters the butt connector one wire may have more corrosion than the other, In such a case one wire will carry more current than the other, and a 14 gauge wire can not safely carry 30 A.

Much better to simply get some more 10 gauge wire than risk having your vehicle catch fire.