When one car is drafting another, the car in front will generally benefit from the presence of the car behind. There may be cases for certain vehicle shapes at certain distances where there is no or possibly negative benefit. Once the two vehicles get very close together there will always be a benefit. Aerodynamic drag on a vehicle arises from several sources one of which is the need for the air to close in and fill the "hole" or lack of car behind the car. Generally a second car, if close enough to affect this flow behind the lead car, will reduce the energy required to pump the air into the hole behind the lead car. This reduction in energy spent pumping air around is seen as a reduction in aerodynamic drag on the lead car, so the lead car can go faster or use less fuel to maintain speed. The cars are not magically connected. What happens is that instead of two cars each having to push air around, there are two cars acting more like one long car sharing the air they push. In the extreme, the second car comes up close enough to the lead car that they effectively become one lumpy vehicle that has a little more drag than one car. The lead car still has to do all the work of pushing air aside and it has the full amount of drag from the radiator, mirrors, wheelwells, etc so the lead car still has most of its drag and sees only the reduction in drag behind. I have extensive experience in auto racing on ovals, and have seen many cases where two cars together are able to run faster than a single car behind. You can see this during every race in NASCAR at Daytona, Michigan, Talledega, California Speedway, Texas, and some other fast tracks.