Put even a fit person who trains only with two feet directly under their center of gravity, and they will struggle with this. Remember, most of all human activity is done on a single leg or staggered stance. If you’re really unstable and lack balance in this split squat position, it probably means you need to spend some more time here.
Once you can do this bodyweight version with great technique and balance, add resistance. Until then, master the bodyweight version. Watch Coach Brandon as he demonstrates the BW Split Squat.