I manage fine with one buckle on the right and a firmly placed belt to keep everything in place.
I started this after ordering kilts from e-bay that arrived larger than expected, making the left-hand buckle useless anyway.
I continued when weight loss made all my kilts rather too large.
Never had any problems of the under-apron slipping.
A new problem that has cropped up, though, is that with kilts less tight at the waist than before, my shirts ride up and look a terrible mess.

By the way, Kinloch Anderson's breacans have a purely decorative strap on the left side; the apron is in fact held in place with a little velcro.

Martin