Can I destroy these adjacent Air Defenses with my Lightning Spells?

Probably

Your positioning of the spells will have to be pretty precise, but it is likely you can destroy both adjacent level 5 air defense towers (1000 hp each) with 3 level 4 lightning spells (390 damage each, 1170 total damage). The uncertainty comes with the fact that lightning spells don't deal all of their damage at the very center. So, some damage potential from the spell may be wasted when stray bolts hit above or below the corner that the two towers meet at.

But it probably doesn't matter if you destroy both, as long as you destroy one

Realistically, though, you probably need to only ensure that one air defense tower is destroyed. Since level 5 air defense towers indicate you might be attacking a town hall 7 base, you don't need both towers destroyed in order to have a successful air attack with, say, dragons. Ensuring that one air defense tower is destroyed should be sufficient for a dragon attack to be successful.