When you say heal you mean revive them when they are at 0 Hp's or simply replenish their life gauge? The later can be done easily especialy on elemental Aeons like Ifrit, Ixion and Shiva. Simply use their coresponding Black Magic attacks on them (Fire, Fira, Firaga on Ifrit, Thunder, Thundara, Thundaga on Ixion and Blizzard, Blizzara, Blizzaga on Shiva) or even their overdrives for greater results. This can be done during battle after Yuna has summoned them so if you want you Aeons at full health, and probably at full overdrive before a big figth, run around for random encounters without Yuna on the party. Instead have a character that usually plays first (high agility attribute or weapon with first strike ability) and switch him with Yuna. This way you can summon an Aeon rigth away. With non elemental Aeons (Valefor, Bahamut and Anima) you can use curative spells but you can only teach them after you have acquired the summoner's soul from your fight with Belgemine
