If you split the damage drags would die slower and burn through stuff before it could kill them. You would just win with low hit point dragons. Honestly if you know the mechanics behind how dragons work its really easy to defend against them. I haven't been 3 starred by an all dragon attack in a long time and my air defense is only lvl 5. Learn to position buildings and they aren't op at all. Low town halls will struggle, but they will struggle against any higher end troops.
No1 use dragons at TH10 so there's no point of having air defense targeting 3-4 targets at one time.