I would allow the use of an environmental trigger to eliminate the second spell.
In general, I favor a relatively expansive scope for the environmental trigger. The subject has been discussed in more depth before, but I can (mostly) sum up the discussion in two points. First, convenience. Second, those published devices built with environmental triggers seem to allow a great deal of leniency in setting the triggering condition. (See the Semita Errabunda "Fire Guardian" and GotF "Crystal of a Hundred Candles.")