It has been made mention of every time I've talked about this. You loose some of the total bonus given to each, but with a duel selection you get a wide upgrade of abilities, at the expense of a single specific aspect with the CAGs. (See values above) Arguably if we had a bomber wing I would suggest we follow the air and naval branches because CAG will never be used for bombing. As it stands the army appears to lack in this department and so it is prudent to follow this specialisation path instead.
These will only be 'one off' researchs.