Besides Yoga, being able to stretch both hamstrings at the same time comes in hand in variety of sports. From Diving to Gymnastics, From Dance to Martial Arts, etc.
The challenge here is obvious. Two legs have to be stretched at the same time, with all the muscles restricting the hip flexion, are now times two. Because of such a high resistance, trying to master double hamstsrings stretch incorrectly, will force the more flexible part of the body to give, and unfortunately that is the lower back. In other words, standard forced stretch method, bring a high level possibility of injured lower back.