An easy way to increase the eccentric (negative) loading is to do the concentric or lifting part of the rep with both legs, then the lowering part with one leg. After you hit failure with the single-leg eccentrics, continue the set by using both legs and a controlled negative.
Alternate legs on each rep. Make sure to do the eccentric portion in a controlled fashion to take advantage of the overloading effect. Don't just let your leg freefall when lowering the weight. Do 4 sets of 8 reps.
Over the last few years it seems that just about every "in the know" performance coach has discussed the conjugate method and why they find it to be the most effective way to train an athlete. By now I think we all know the definition of conjugate or concurrent periodization and how it involves simultaneously trying to raise multiple qualities.