The problem with that setup is that it will only be efficient at a certain wattage. Let me clarify what i mean. Say you have a cpu that runs default at 1ghz. You want ot overclock it. You put your setup all together and make sure that the radiator(s) can disipate all the heat in easily enough time before it gets back to the cpu. Because you know that if you didn't, the water will steadily increase.
Now what factors did you have to take into consideration in order to do this in the first place? CPU speed, TEC wattage, water flow, fan speed and c.f.m., at the very least.
Now say you barely adjust just one of the factors, your setup is off and you must recalibrate everything. You could do it, but it would be very time consuming.
your set-up is wrong and it'll neutalise the peltier by averaging the cold plate water with the hot plate water
if you want to reduce the risks of a Peltier breaking down, put another peltier next (NOT stacking) to your first one (your need a larger cold plate and water block to cover both) and the reserve up can be turned on by a thermostat after temp rise beond X degrees C, of course the spare Peltier need to be powered by an independent PSU