First off, Split Squats are not lunges. This is a stationary movement that's staggered stance in the sagittal plane of motion. The Split Squat will require proper hip, knee and toe (particularly the big toe) mobility in order to execute properly.
If you feel any pain or limitations within the movement such as knee pain in the back leg or limited mobility in the big toe that causes you to compensate by rotating the heel outward, then you will need to address this before you begin with the exercise. In some cases this can be quickly accomplished by foam rolling your Quadriceps and massaging the arch of your foot, then moving into active stretching for each area. In this example, you would perform a Side Lying Active Quadricep and Kneeling to Tall Kneeling Stretch.
Begin with 3 sets of 10-20 repetitions for each leg before increasing the intensity of the exercise with weight or progressions.