I have used Addidas Sport Glasses with a bifocal insert (snaps in right behind the regular, non-prescription lenses) for years, and really like them. I looked for them online to give you a link, but I didn't find my exact model. It appears now that you can get a similar product with prescription lenses, even bifocals, etc. as the main lens. What I like about mine is that with a prescription insert that snaps in behind the regular lenses, I can switch out the regular lenses to meet the light conditions: Clear for nightime, yellow for overcast and grey for bright sunlight.
I got them from my optometrist. I don't know if they are made specifically for motorcycling, but the lenses are shatterproof, etc. the earpieces are adjustable to vary the tilt of the glasses, but still very rigid, so it's easy to get them on after putting on my full-face helmet. The suggestion of wearing regular prescription glasses with a sunglass in fromt of it means you've got two sets of earpieces. That does not sound very comfortable in a helmet. I really think you should check with your optometrist and see if he has an Addidas sport glass catalogue.
I'm going to look some more, and if I find my glasses, I'll post the link for you.
OK, I found a link to info about Addidas sunglsses that includes the "optical insert." Check it out:
http://www.heavyglare.com/adidas-sunglasses/