As far as I know, only one example of a three-part Monstrous Cavalry exists: Naestra and Arahan riding Gwindalor.

Normal Monstrous Cavalry are damaged as a single model that uses the highest Wounds characteristic of the rider or mount, but how does that work when there are two separate riders? The twins each have two wounds, where the eagle has three. Do the twins both get an extra wound off the eagle? Is everybody just treated as one model with three wounds, making the Conjoined Destiny rule slightly worse than if they were on foot? Or would it be one model with four wounds, because "the rider" is two riders and between them they've got four but the MC rules only cover "the rider"?

I can't be the first person to think about this; Great Eagles have been MB since 8th dropped. Or does everybody just use Cethin-Har instead?