In judicial practice, back-to-back life sentences are two consecutive life sentences given to a felon. This penalty is typically used to prevent the felon from ever getting released from prison.
This is a common punishment for a double murder in the United States; this is effective because the defendant may be awarded parole after 25 years when he or she is eligible, and then must serve an additional 25 years in prison to be eligible for parole again. It also serves as a type of insurance that the defendant will have to serve the maximum length of at least one life sentence if, for some reason, one of the murders is overturned on appeal.
Other countries either allow multiple concurrent life sentences which can be served at the same time (e.g. Canada), or allow multiple consecutive life sentences with a single minimum term (e.g. Australia), thus allowing earlier release of the prisoner.