It's probably the same as making a twin turbo setup as in you will have to adapt it to have the same characteristics of twin turbos but instead with 1 turbo. Same principle goes for this. Doubling the radiatoreffectiveness value should have the same effect as adding another radiator anyways.
No, that's currently not possible (I guess you are aiming at having two collidable radiators?) You can probably span the rad beams across the whole car, making both sides damage the same radiator.
