How can I specify that two teachers can teach the same subject for the same class, but just one be selected when the timetables are generated?
I think it is impossible. The FET notation is that the user chooses the exact teachers for each activity, and these teachers will be allocated at a specific time for that activity. FET cannot choose only one of them.
Maybe tricks are possible, like: consider a real teacher=a FET room, and preferred rooms for the activity (A) are T1 (real teacher 1) and T2 (real teacher 2). When FET places activity (A), it will choose a FET room=a real teacher.
Or specify no teacher for the activity (A) and add later the teachers.
I am not sure these tricks may work in your case.
Maybe the best idea for you is to choose the exact teacher for each activity and hope that FET will find a timetable. If not, you can move some teachers and try again. So, you will take control of planning and let FET do the timetabling.
FET does not do course planning.