Yeah you have the fuel tansfer problem. Your truck does not have the a switching valve just a pump module inside each tank. To disable a tank disconnect the power at the tank so you dont accidentally empty the contents on the ground and create a fire hazard, and find some studs like those in the pic below. A couple minutes with a hacksaw and grinder and you can turn them into plugs for the fuel lines, note you will need 2 different size studs for the send and return lines. Don't simply connect the 2 ends together or you'll create a fuel delivery problem with the other tank.