Do one leged leg presses,super-setted with the one legged exts.,......do the exts. first to pre-exhaust the legs,then after a few sets of each,do walking dumbell lunges for a few sets to finish them off.
Also,doing step-ups on a platform or bench might help........do one leg for the amount of reps you are shooting for,then do the other one.......do not alternate left and right per rep,finish say 15 reps on the right,then do 15 reps on the left and so forth.