They're not double-sided, but my wife got me a pair of the Nashbar ATB panniers for christmas. I'm lovin' 'em so far!

One of the nice things about them being two separate bags is that you can use them one at a time. I loaded one of them up with work clothes, but then didn't ride to work for a few days. With a one-piece bag, I would have had to drag the clothes around everywhere I went, unpack them, or not bring the bags. Since mine are separate, I just used the other bag.

I also like how easy they go on and off. I take them off of the bike when I get home and unpack them inside. When I want to go for a ride, I pack them inside and then bring them out to the bike. I like not having to pack the bag on the bike.