I have duel tanks on my truck.Outside a leak , or more hwy driving on one tank,is it possible for a tank to perform better than the other in the gas mileage department?If so,what would cause it?
It’s ‘dual’. ‘Duel’ means the two tanks are fighting. 
Do you manually switch from one tank to the other?
I think you have the two most obvious explanations in your OP. I suppose it’s possible that the line from one tank might be clogged and require more power to pump the fuel, but that seems a little farfetched.
Also, if you start off with both tanks full, and draw from one only to start, you’re carrying the weight of the other tank. When the first tank empties and you switch to the other, you are then carrying an empty tank. So I suppose you might get better mileage from the second tank. But I don’t know how significant that would be; it would probably be swamped by the effects of weight for traction in the cargo bed.
From the Department of Farfetchery: How often do you drive the truck? Is it possible you filled the tanks far enough apart that you got gas mixed for one season in one tank, and gas mixed for another season in the other tank? How consistently do you see the mpg difference?
How are you measuring the mileage you get? If you can consistently fill each tank to a reproduceable level, and use the pump reading and odometer reading to calculate an average from the previous filling, it should all be correct, excepting the very reasonable points folks made above. But if you are trying to get estimates on, say, every quarter tank while the tank is emptied, then the inaccuracy of the gage in each tank would contribute error.