He gets an equal increase to his damage from Aim and Cunning; as in: each point in Cunning will give him 0.20 DPS and each point in Aim will give him 0.20 DPS.
However, in an ideal situation you would use both stats. This, because a main stat also gives additional crit chance, however this is on a diminishing return scale. What this means: 700 Aim + 700 Cunning = same amount of raw DPS, but more crit chance, than 1400 Aim and 0 Cunning, for instance. Try to balance out both stats for maximal returns.
Thank you very much for your anwers, i was thinking that, so you think it would be nice if i had the all of the Assasin droid gear made, but instead of going aim with the barrels for both OH and MH i should just make the 63s barrels cunning based and keeping the mods and enhacements also cunning? and btw is there diminishing returns for main stats on companions?