Does illustrator have the equivalent of an "Add" blend mode? This would mean that
(R=0, G=128, B=255) + (R=255, G=128, B=0) = white
There isn't an equivalent of Add/Subtract or Linear Dodge/Burn in AI, unfortunately. Using "Lighten" can sometimes get you there, but in almost all cases requires a third layer, set to Lighten or Color Dodge, to "fill in" the remaining channel.