I agree, it does sound slightly ludicrous but I can only guess that over time there will be synergies in back office functions such as finance, HR, IT, maybe even engineering and that is where savings will come from. At least that is how other private companies do it!
In practice what you suggest should happen however National Govts are protective of their national carriers be it UK or Spain. Hence why I believe lots of what they want will not happen easily or without great cost.