The weight of two guns of smaller caliber would be more than one gun of higher caliber, which would slow the tank down for one thing. (Most of the time)
Yeah I guess the tank would become too heavy to use as an MBT but I'm still surprised that those wicked soviets didn't try something like that during the cold war or even earlier. Atleast I haven't heard of any designs with a moveable turret that has 2 cannons.
I'm not necessarily talking about modern day designs.
Plus you'd need an extra loader and gunner, which means more people risking their lives.
I don't think you need an extra gunner for a doube barrel turret and you can use autoloaders instead of manual loaders ( which isn't as effective but still possible ). Plus said wicked soviets wouldn't really care about loosing one more loader in a single tank.
EDIT: I understand what you mean. I wasn't clear enough in my previous post : I was wondering why nobody designed a tank with a turret that has dual cannons ( see mammoth tank).