Reminds me of being a kid with my Honda SL70, an overnight camping trip on our motorcycles. We would carry a few extra gallons also strapped under a backpack on the rear.
The only bad part was when my friend dumped it and was trapped underneath the bike with the gas leaking onto the hot exhaust...we laughed but he was like some freaking wild rabbit trying to get out from under it!
Not a bad idea, looks good. I would think that the extra tank would leak a little from expansion and get fumes on the backpack, which could ruin your food and/or water, so I would carry food separate from the fuel container.
without the stuff above the tanks it looks like a no-brainer.