You could connect them both to the Phantom's Wi-Fi network at the same time - BUT only one device at a time can actually connect to the camera. The second one will get a "connection failed" error. It would seem the little "video server" app built into the camera only supports one client session.
EDIT: since both devices can be on the same Phantom Wi-Fi network, it would be great if you could just use AirPlay to mirror one device's screen to the other. Unfortunately, Apple doesn't support iOS-to-iOS mirroring. You'd need some device that's actually an AirPlay receiver, like an Apple TV box. There used to be some AirPlay receiver apps for the iPhone, but looks like Apple pulled all those from the App Store back in 2012. Maybe if you jailbreak your phone...
