I wouldn't do it. The absolute most important aspect of any pannier is its mounting system and there is virtually no information at all on the website about it. Any expensive pannier will be rendered useless if the mounts are not strong or well designed. I have looked at a lot of backpacks that convert into panniers and ultimately they are a compromise both on the bike and on your back in terms of ruggedness and comfort. Also, those bags are freaking HEAVY at 5 pounds! Most panniers weigh much less than that and will probably be easier to use overall.

Instead, I would just carry an ultra light backpack that fits inside your normal panniers. I don't ever need 60 liters worth of space when I go backpacking because its just too much weight. I usually carry a nylon day bag with me on tour for waling around or hiking but if you want to go on longer treks I would suggest something like this: http://www.rei.com/product/778467

A really nice ultra-light backpack, that can also be a 60 liter model if you want one, can be had for under $100 and then you can buy normal panniers that will be much lighter and cheaper than that set. You will also not have to compromise when it comes to either sport. The weight penalty for carrying the backpack will be offset if you can get a waterproof model that can double as a dry bag to carry your tent and sleeping bag on your back rack while you ride such as any of these models: http://www.ortliebusa.com/cartgenie/prodList.asp?scat=3