I just came back from a backpacking trip, and I carried my camera in this:
http://www.amazon.com …eywords=m-rock+camera+bag
I only took one lens with my 5d, but I really liked using this holster bag. I literally clipped it to my pack with a carabiner, and it was easily accessible. When we were in camp, I clipped my camera strap to the bag and carried it sling style. It's small, but there was enough room for a couple filters, batteries, cards, brush, etc... I would recommend that you get a lens wrap for your other lens, and pack whichever one you are not using in your main pack. You could also get a smaller lens pouch to clip to your bag as well.
You might also look into Clik-Elite as I think they have good options also.