We have seen that real engines always discard substantial amounts of heat to their low temperature reservoirs. It seems a shame to throw this heat energy away. Why not use this heat to run a second engine, the low temperature reservoir of the first engine serving as the high temperature reservoir of the second?

The efficiency (η) of a heat engine is defined as,
η = 1 – T₂/T₁
Here T1 is the temperature of source and T₂ is the temperature of sink.
The efficiency of an engine depends upon the temperatures between which it operates and is independent of the nature of working substance. The efficiency of an engine will be 100 % (η = 1) if the temperature of sink (T₂) is absolute zero. Since absolute zero is not attainable, even an ideal engine cannot have efficiency as 1. This signifies that, it is impossible to extract heat from a single body and convert the whole of it into work.
In the case of real engine, if we use the discarded amount of heat from low temperature reservoirs of the first engine serving as the high temperature reservoir of the second, then after certain interval, the temperature of the sink will tend to absolute zero. But, which is not possible. That is why; you cannot use the discarded amount of heat from low temperature reservoirs of the first engine serving as the high temperature reservoir of the second.