There's actually a silver lining to the lack of connection between the controls on both sides of an Airbus. It is that, unless they hear "DUAL INPUT" (or "I have control" if things go south far enough), the trainees can be totally certain that it was them and only them at the controls all the way through. In this way, it's a bit more clear-cut what works and what doesn't. In an aircraft with proper dual controls, some instructors may do subtle interventions on their side without later explaining that to the trainee - and, hence, the trainee can be left with a somewhat flawed impression of what needs to be done because he doesn't factor in that subtle, unmentioned input from the instructor.
That's the glass-half-full viewpoint though. The glass-half-empty one is that teaching and intervention indeed become more difficult and are well worth some dedicated discussion during LTC/TRI training (and, if you ask me, during every Airbus pilot training as part of the pilot incapacitation topic). The standard rules for taking over in the event of suspected incapacitation are not always safe enough close to the ground as you might not have the time to wait for a response to two consecutive deviation callouts. Conversely, if you always take control as soon as anything deviates from normal, that may be detrimental to the trainee's learning. It can be a tough call - and one that only comes with experience.