Can Peltier devices be cascaded?
Yes, and this is regularly done. However, there are limits to what you can achieve, based both on the limits of the individual devices (minimum and maximum temperature) and effects such as the total thermal resistance through the stack. Eventually you get to the point at which the "reverse leakage" of heat through the stack (which rises with the end-to-end temperature difference) equals the stack's ability to remove heat.

Another problem is the relative inefficiency of Peltier devices. Typically the heat flux coming out of the hot side of each device is on the order of 3 to 5 times the heat going into the cold side. As you stack devices up, each one needs to be that much larger than the previous one, leading to problems with sheer size (which also gets back to the heat leakage problem).