I have done my fair share of pull-pull systems and i have never had a problem if one cable goes slack when the other is pulling the control surface...in fact i offset my horns aft of the center line to gain control surface movement and it increases the leverage of the sytem and therfore takes less power to run a 5 inch aileron up to aprox. 75-80 degrees of one direction of travel. Yes the cable that is not pulling the surface goes slack but the airflow is pressing against the surface and against the pulling cable.
Another trick we have learned is to eliminate the crimp on type cable locks which can easily be crimped on to hard and cut the cable and eventually the cable will break. Loop the cable thru your horn and wrap it about 3 to 4 turns back up the cable running from the servo and then melt the plastic coating from both cables now together with a lighter or small torch. Never had a problem with the cable breaking then.