https://github.com/unhammer/lttoolbox/tree/df-intersection contains a stub function in transducer.cc called "intersection". The main loop needs implementing.

This task is not to do the actual intersection, but to traverse the states of the two transducers in parallel, only following an arc from this transducer if there is a matching one from the current (set of) live states in the other transducer.

See also: http://wiki.apertium.org/wiki/Task_ideas_for_Google_Code-in/Intersection_of_ATT_format_transducers

Come on IRC to discuss this task http://wiki.apertium.org/wiki/IRC