Hi, I agree with James.
I just came through Eastern Europe, Middle East, Saudi, Iran and Central Asia, and only once did I have to use spare jerry cans. I was carrying two 5l plastic ones and strapped them on the front sides of the panniers.
My friend strapped one of his on the bike, above the exhaust pipes, on the tine plastic rack you know. BAD IDEA. It got loose and drifted off and actually leaned against the exhaustpipe. Have a guess if we were worried when we saw the melted plastic...
If you still want to take 2 10l tanks, I talked to a guy at Touratech who built aluminium frames that he mounted on the bottom of the panniers and just slid the tanks in.