Twin lens Mask = a left and right lens.
Image of Twin lens mask (Mares X-Vision Liquidskin Mask)
Single Lens Mask = one single piece of glass across the front of the mask.
Image of Single lens mask (Tusa Concero Frameless Mask)
It’s purely a personal choice which type of Diving Mask or Snorkelling mask you feel more comfortable with. For snorkellers with larger or previously broken noses the single lens design may be preferable.
‘Off the shelf’ prescription lenses are available from some manufacturers for their ‘twin lens’ masks although depending on your prescription requirements you may need ‘made to measure’ lenses and twin lens masks are generally considered better for this.
Do you prefer using Single lens of Twin lens masks?