I have an Arkel commuter as well and it is a great pannier (near-waterproof, great organization, enough room for commuting, etc., etc.). It absolutely sucks as a backpack (think single shoulder strap like a huge purse). The Arkel Bug or Switchback are your best options if you want a single bag solution for a backpack & pannier. Having said all that, if comfortable backpack use is critical then I would follow no1mad's suggestion of getting a real backpack & store it in a basket on your bike.

The problem is that the qualities that make a good pannier do not make a good backpack. No matter what you do you're going to have pannier mounts digging into your back and/or shoulders.