If you control more than one Ancient Adamantoise, you choose which redirection effect to apply. You can't divide damage dealt by a single source between them. For example, if an attacking creature would deal 8 damage to you and you control two Ancient Adamantoises, you'll choose which of those Ancient Adamantoises will take that damage. You can't have 4 damage be dealt to each Ancient Adamantoise, and you can't choose to take the damage yourself.