Replying to Oakley single lens vs. double lens

I've been searching for a cheap crowbar lens for night skiing and I've seen so many of these crowbar single oakley lenses going on ebay for really cheap. I know if you buy any pair of crowbars from a store or online or whatever you get the double lens. I was wondering what the difference is between the single lens compared to the double lens. I'm sure the double has an advantage, but will the single still work without fogging, and blocking out light? Also are the lenses on ebay even legit oakley products? If you have any info please let me know, thanks. +Karma