Two weaker peltiers vs one stronger peltier for cooling?
If the water is stagnant, it's better to use two modules, one on each side of the container.  If you wish to use a heat sink for faster cooling, place it on the hot side as this will remove heat from the module and thus allow it to operate more efficiently.  From experience, I find that working the module at about 80% of full rated value is best for longevity. If speed of cooling is not a major concern,  connect the two modules in series across 12 Volts and each would receive the same current (but lower) at 6 Volts each and thus you would use only 1/2 the power as in the original design.