Back to back life sentences are prison sentences that run consecutively one after the other. Life sentences are given for various first degree felony charges; mainly rape or murder. If a defendant kills more than one person during a crime spree, it is usual for each instance of murder to be treated separately and the same amount of time given as a sentence for each instance.
Back to back life sentences are the result of people's idea that a crime committed must be brought to full account individually and it would not be fair for a person to only serve time equal to one murder sentence. It is up to the judge in each case to decide whether the time is served back to back or if the sentences can be counted at the same time.