I have a similar situation to this one with a 120V Stenner (already purchased) and a 240V pump. Rather than do what has been suggested there, why can't you install a 240V relay that is normally open. When the pool pump is running, it will close the connection which can be a 120V circuit running the Stenner and a timer for the Stenner. When the Stenner timer AND the pool pump are on, the Stenner would run. When EITHER the pool pump or timer is off, the Stenner will not run.
Anyone know of a relay that could be used to do this?