If tank pressure is sufficent then a second cooler will add 30-50% more liquid flow than a single cooler. Do not expect double the flow. That said, temperature and tank pressure are the largest limiting factor. If its too cold you will not gain much. If its a 60 degree day you will see a big difference.

For that size bar you should at least have had dual coolers on it to start with. You would be better with 2 tanks.