Back-to-back ticketing is an airfare booking ploy used by savvy fliers to circumvent high fares from airline’s fare system by purchasing two sets of R/T tickets for either one or two flights, while making use of knowledge that Saturday stays usually are cheaper than midweek R/T flights. There are two slightly different examples as to how this could work:
Let’s say you live in Philly, and have two prospective job interviews in Boston; both are midweek ($$$), and they are on two different weeks. So you need to make two costly midweek trips, PHL-BOS. Because you know that Saturday stays usually make a flight cheaper, you can book a trip that leaves PHL for BOS on Monday of Week 1, and returns Friday of Week 2. Then you book the exact opposite trip, leaving BOS back to PHL Friday of Week 1, and returning to BOS Monday of Week 2.
However, you go for your first interview on Week 1 using PHL-BOS of the first leg of Trip 1 and returning BOS-PHL of the first leg of Trip 2. Then, for the next week, Week 2, you use the second leg of Trip 2, PHL-BOS on Monday of Week 2, and return on second leg of Trip 1, BOS-PHL on Friday of Week 2. It sounds more complicated than it really is; I tend to have a knack for that.
The other, less likely way that back-to-back ticketing has been used is when a buyer, only intending to fly once, buys two round trip tickets, but flies one leg of the first ticket and the one leg of the second ticket only. It was done by knowledgeable fliers for a time, but is much harder now, because the price for the same set of arrival/departure destinations do not vary as much in price like they used to, and also because now most airlines now require a traveler to fly each leg of the ticket in order as purchased; you can almost never fly the second leg of a round trip ticket without first flying the first leg. Though either strategy is not illegal, many airlines will penalize you if they catch on, by perhaps confiscating tickets or, more likely, cancelling frequent flier status. Be careful!