Do you mean running a parallel pair to increase ampacity compared to a single wire (although dividing by 2 would put you back where you started
)? If the two are close together, then they may have to be derated somewhat to maintain a safe temperature.
Since power dissipation in a given resistance is proportional to current squared
, current only needs to be divided by the square root of 2 to halve the dissipation. In reality, you won't even have to derate that far as the two wires have a greater surface area than one of the same size; treating the pair as equivalent to a single wire of double the cross-sectional area (or with an AWG number 3 lower) should be quite safe (a bit more current may be OK as the two smaller wires still have more total surface than the bigger one, but I can't give precise or official data).
If it's just to reduce voltage drop, then you won't need to derate further.
This is just my understanding of the general principles; I'm not involved in distribution specifically, so there may be some detail I don't know about.