If there is no opposing air force, that is mostly true.
14 Interceptors @ 96% condition has 48% SBDE. So, two stacks of 7 each would do almost twice the attack damage while taking more damage initially since their would be two attacks per time period. Since you are doing on average about twice as much damage, you would destroy enemy units quicker.
So, having 15 of each in one stack doesn't necessarilly do the most damage on average. So, for anything above 14 planes each, two stacks of planes is often better. Also see substitution below. It might be better to use less of each plane type and use substitution.
MORE HIT POINTS: While you have more hit points that really doesn't help since you will lose an interceptor for every 20 points of damage to your interceptor group, a tac bomber for every 20 points of damage to your tac bomber group and a strat bomber for every 30 points of damage to your strat bomber. By having two attack groups you could take as much as 39 points of damage to your 2 interceptor or 2 tactical groups before losing one of them. Your strat bombers could take as much as 59 damage points before losing your first strat.
SUBSTITUTION: If their is no significant opposing air force, you can also substitute a plane(s) of same type with more hit points for one that has received damage during the battle. It takes more time but may save some planes.
STRATEGY: Often you can destroy some units of the stack. Then wait for day change. Then continue the attack after day change. Your planes can heal 15% of their damage. Since the defenders have fewer units left, your planes are stronger and probably won't take as much damage since you have destroyed some of the enemy units.
