To the OP.. 335i's (and thus I presume 320i's) can have a maximum of two FOB's linked to the car at any one time. The idea being if you loose a FOB (or have one stolen), when the dealer provides a new FOB and this is synced with your car (as I did with my own), the "lost" key will no longer work, without first being initialised (which required the other key!)
So whilst the owner of your stolen key maybe able to unlock the car with the key they have, they will not be able to start it.
And, we are talking crooks here, they are that f-ign dumb, they probably dont even know how to remove the emergency key from the fob.
My advice, have your insurer replace the key and move on, not worth the effort and potential damage of having all new locks installed!
2012 Audi Q7 3.0TFSI - Mineral White
2010 M3 E90 - Alpine White