Gods have always had the ability to be in multiple places at once, although there were four official, Divine rules they had to follow. There was also one other unspoken rule that they all followed - you had to be fully on Olympus during council meetings, meaning no manifestations anywhere else and no sending a manifestation to the meeting. But Poseidon has a reason to be breaking the unspoken law, even if he hasn't told any of his family about the turmoil going on under the sea.