Multiple TEC's means multiple waterblocks.

One high wattage TEC (100watt) will provide the same cooling power as two TEC’s at half the wattage (2x50=100watt) rating when cooled by a single water block

One high wattage TEC (100watt) will provide less cooling power than two TEC’s at half the wattage (2x50=100watt) when cooled by a two waterblocks.

Since there are two TEC’s taking up twice the space, this enables us to attach two water blocks to the hot side of the TEC's. This is desirable because it halfs the C/W of the hot side. Lower hot side C/W’s result in lower hot side TEC temps which in turn results in lower cold side temps. Here is an example.

One 120w TEC will cool a 50 watt load to 14.7c while drawing 50watts of electricity when being cooled by one waterblock.

Two 61w TECs will cool a 50 watt load to 5.9c while drawing 50watts of electricity when being cooled by two waterblocks.

Four 61w TECs will cool a 50 watt load to -2c while drawing 50watts of electricity when being cooled by four waterblocks.

Multiple TEC's means more heat can be moved.

The next benefit of using multiple TECs is to increase the maximum heat that can be transfered. If you’re trying to cool a 150 watt load and the most powerful TEC you can get your hands on is a 100watts, you’ll be getting yourself into trouble. However three 100 watt TEC’s (300watts) will get the job done.

Multiple TEC's means more efficiency.

The last reason for using multiple TEC’s is to increase efficiency. A TEC is more efficient the lower you run the input voltage relative to Umax (max input volts). As you lower the input voltage the TEC can move more heat relative to electricity used. Decreasing the electricity also leads to lower Hot side temps for any given hot side C/W

One 120 watt TEC running at Umax(24.6V) can move 120watts of heat to ambient while using 251watts of electricity. The total heat to be dissipated by your cooling system would be 371watts.

Four 120 watt TECs running at 3.2Volts(13% of Umax) can move 120watt of heat to ambient while using 17watts of electricity. The total heat to be dissipated by your cooling system would be 137watts.

Draw backs of low input voltages

The draw backs of low input voltages relative to Umax is that as the voltage is decreased so does the maximum temperature difference between the two sides of the TEC. Decreased temperature difference limits the lowest loaded temperature achievable.

One 245watt TEC running at Umax can achieve a temperature difference of dTmax (67c).However this same TEC can only achieve a temperature difference of 25c running at 20% of Umax.