lower body training
Hi Jane! There is no set rule as to how to train your legs. If you find that your body responds better to two separate leg workouts(each targeting different leg muscles), then there is nothing wrong with training them twice a week.
To answer your question more specifically, the reason that legs are usually lumped together into one workout is because most leg exercises target many leg muscles in just one exercise. For instance, a squat will work the quads, hamstrings, glutes, and buttocks. Not only that but variations of squats will also target the outer and inner thigh area. And for calf development(which is very much directly related to your genetics), a few challenging sets of calf raises once a week in addition to your challenging weekly cardio workouts(which VERY MUCH use your calves)is truly plenty to keep them fit.
Another reason that leg workouts are lumped in one workout, specifically as it relates to home exercise workouts, is because without access to specific machines and gym equipment, you are limited as to how many exercises you can effectively do to challenge each of the leg muscles separately. For instance the leg extention machine specifically targets the quads, the leg curl machine specifically targets the hamstrings, and adductor/abductor machines specifically targets the outer and inner thigh muscles like no other exercises can. Hope this helps.