When doing an exercise that works your body halves separately, like lunges or side planks, do your reps on the weaker side first. Even though the sides of your body are moving independently, both sides of your body are still working. You want to give your weaker side a chance to perform reps cleanly with good technique before tiring out. Generally, the leg you don't want to lunge on is your weaker leg. Don't avoid it! Start with it.