Am I understanding this correctly?
Two wires of half the cross sectional area at DC don't quite give you double the current rating. Take for example AWG 24 (84 ohms per km) and AWG 21 (42 ohms per km).
AWG 24 allows a maximum amps for power transmission of 0.577 amps whereas AWG 21 (close to double the cross sectional area) allows 1.2 amps. 2 lots of AWG 24 only allows a current of 2 x 0.577 = 1.154 amps. Additionally you would need to stop one wire heating the other one up so this means two lots of AWG 24 would need to be spaced apart a little bit.
This makes it somewhat inconvenient of course.
However, I would think that going from 1.5 A to 2.5 A by using 2 lots of AWG 20 would be OK and, of course, if the current passing were a high frequency superimposed on DC then you might be significantly better off given the skin effect seen by AC currents.