Executive producer Martin Moszkowicz has shot down speculation that The Mortal Instruments: City of Ashes and The Mortal Instruments: City of Glass will be filmed back-to-back in 2013.
City of Ashes is slated to begin production in the fall with Harald Zwart returning to direct.
— Martin Moszkowicz (@mmoszkowicz) June 13, 2013
The back-to-back filming has been mentioned a number of times, as a means to save production costs, etc., but it sounds like ultimately Constantin Film felt it best to focus on one film at a time.
Of course this could mean a number of things. Constantin Film could be waiting to see how well The Mortal Instruments: City of Bones fares at the box office in August, Zwart could want his full attention on one film at a time or hasn’t yet negotiated a contract for the third film, or production on City of Glass could actually begin in 2014, which is technically “next year.”