They aren't adding two-player flying mounts, because there is no way to dismount IN MID-AIR. Imagine if the person controlling the mount flew into the air and went AFK. You couldn't dismount as you would from a ground mount. You would be stuck in the air with no way to get down.

In WoW, you could actually dismount in mid-air, and fall all the way back down (usually killing yourself from fall damage). That isn't an option in FFXIV for flying mounts, so the passenger would literally be stuck unless they added in a parachute feature.