"Wrong" digital code output in pre-stage brings "wrong" reference added to multiplying DAC so as to "wrong" digital code still, but addition of two "wrong" code gives wanted correct code.
the additional code (additional comparator) avoids the input of the next stage to be saturated (larger that |Vref| ) in presence of comparator offset. draw the characteristics for 1b and 1.5b stages!
1 The digital output is overlap each stage, then they can be added.
2 The decision line deviation each stage must less than ±1/2(1/Gi - 1/Ni)Fs that can guarantee the correction result right.
Where Gi is i-th stage gain. Ni is number of i-th stage ADC code.
Detail you can reference CMOS data converters for communication p235-239