In export_descr_units.txt is an entry for an unit which only has a single model.
In model_battle.txt there is an entry for the model which dictates a single CAS file but multiple textures.

I hope you understand now the hierachy. Or another try:
export_descr_units -> 1 unit entry = 1 model
model_battle.txt -> 1 model entry = 1 CAS file + multiple textures

So if you want a different model for the same unit, then you have to make a new unit entry. You can make the unit entirely identical except for model/texture, but for the engine the units are still different. So you cannot merge them.