I can't see what you'd gain by having an internal 2nd strap. The second strap on a kilt is truly decorative only, and, frankly, all kilts hang better if they don't have that second strap.

The only kilts that actually need a second strap on the apron edge are dancer's kilts, and it's only to add some control to the kilt edge during dancing. Highland dancers don't wear kilt pins, and they do a lot of leaping around. The second strap just keeps the apron from flapping around quite so much.