I'm not sure you can really kill either of them. What you want to do in the throne room is pick one (I prefer Madae myself; she doesn't have a blue flame shield up) and just keep smacking away until Isair decides to fall back to the pool room.
You clean up any critters left, then exit for an auto-jump to the pool room and the final confrontation. This is a bit easier if you've tainted the pool. Shortly after this, Orrick should appear with a message.
Nightowl2
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