Erm, kinda yes/no? Yes, the eidolons stack, but the thing is Character Atk vs elemental Atk are calculated separately. Elemental attack (like on apocalypse) becomes FAR more useful than character atk with your weapons grid. There's a post that talks about the actual math (with proofs), but basically take it like this: elemental attack is better. It actually becomes worth MORE as your weapons grid rises. Almost exponentially more.