It seems like in a tank battle the tank that is able to put more shells on target would have a much greater chance of survival, especially with the implementation of autoloaders it seems like it would be a no brainer to have a tank with multiple cannons. So why was it never implemented?
This is more of an engineering consideration than a historical perspective. In WWI the British mark 1 had two cannons, but neither were mounted in the fashion of a modern turret that has a 360 degree field of fire. They were also only 6lb cannons. For comparison, a modern tank cannon, say the barrel of the Rheinmetall 120 mm gun weighs 1,190 kg (2,620 lb) with an additional 3,317 kg (7,313 lb) for the mount. To install 2 or more would require a tank with a minimum of 20000lbs of cannon. This larger tank would also be larger to have room for the cannons, thus requiring more armor and a larger engine. Pretty soon you have a monsterous size tank that has an impractical size that cannot maneuver and serves as an easy target.
Another practical consideration is simply the expense. If you build a smaller single turret tank, you can build more of them for the same cost as a larger tank with multiple cannons. More tanks on the field means better redundancy in the event that one is destroyed. This was demonstrated in WWII by the M4 Sherman tank. The USA was capable of mass producing these tanks, which were inferior to several of their contemporary German counterparts. While the German tanks like the Tiger 1 were superior individually, they could be overwhelmed by superior numbers.
Tl;Dr Weight makes such a design impractical, unmaneuverable and ultimately more vulnerable.
Knocking out a tank usually requires one well aimed shot that penetrates the armor. One big gun is better at penetration than two smaller guns. Additionally one large gun will have a longer range, and the recoil from the other gun will not reduce the accuracy.
Multiple guns are for when you need to throw out a lot of metal to generate a shotgun type effect. This is usually for fast moving targets like aircraft or multiple soft targets like infantry.
Usually only takes one direct hit from a gun to knock out a tank, but adding guns- in the early days of tanks most had more than one gun- made the tank more vulnerable and added unnecessary cost and complexity. More than one prototype tank was abandoned as a design because the tank became unwieldy and unreliable under the burden of all the additional guns (http://en.wikipedia.org/wiki/T-35 http://en.wikipedia.org/wiki/SMK_tank) or simply yielded no particular advantage- when the Germans conquered France and seized their stock of B1 heavy tanks the Germans removed the 75mm howitzer mounted to the front of the tank and welded over it with armor plating for a reason.
The closest actual design that fits the OP's criteria is the Panzer VIII Maus which had a 75mm KwK cannon (the main gun of the Panther) as a coaxial mount of the main gun. As the other commentators have noted, putting in more firepower entails considerable weight and size penalties. These penalties kill maneuverability and increase its viability making such a tank of limited use.