I have two heroes stationed at the banks. What does the economic practice skill do exactly, and what..

Economic Practice skill will give Exp points to the hero, so they can raise their level. You will get more points depending on the level of the skill and the current level of your banks money production.

The cool down is just a period of time where you cannot use the skill again. You have to wait until the cool down finishes to get more Exp this way.