I remember this one glider pilot who duplicated all instruments in his glider, so that he would never fail a glider competition day for technical reasons. And yes, he also had two yaw strings, this guy was r-e-a-d-y :)
But that's no why that particular guy had two strings :). He just wanted to be double sure that his aircraft will perform at all times. I think he even had 3 certified flight recorders since misconfiguration or other failure of the certified flight recorder is the most common technical problem during a competition flight (you might not be aware that this little box is faulty until after you land). Actually quite a few "serious" glider pilots carry a second flight recorder.