The design of the SR-71 creates requirements that affect the design and operation of the KC-135Q tanker aircraft.
Under most circumstances, if the KC-135Q offloads enough fuel for the SR-71 to complete its mission, then the KC-135Q will be low enough on fuel that its options for a landing site are very limited.
More importantly, the SR-71 mission should not depend on the reliable transfer of fuel from a single tanker. There are many issues that can arise all along the way from aircraft generation to actual in-flight refueling. And experience has shown that these issue do arise and the reliability of a single tanker is too low to hang the entire SR-71 mission on.
Maintenance and operational crews could always generate one or two “ground spares” in case the primary aircraft has a problem and cannot take to the air. But since the SR-71 usually has a specific mission with specific timing, delays could mean mission failure.
All of this, and more, led to the reality that Beale AFB Tanker crews, unlike all the other USAF tanker crews, had to be proficient in formation flying. The SR-71 needs inflight options. For all SR-71 missions, it turns out, either a two or three-ship of tankers is sufficient to ensure adequate reliability of fuel offload.
These missions are planned based on the possibility that a single tanker may not make it to the refueling point, leaving either one or two tankers to provide all the needed fuel.
Flying any aircraft in close proximity to each other creates a high collision hazard. Pilots and crews keep a constant and wary eye out for each other to help avoid collisions.
The speed and altitude of the SR-71 leads to small cockpits windows and a pilot who is wearing a space suit. The SR-71 pilot’s lack of visibility under the best of circumstances means that a typical formation you might have seen in old WWII movies is unacceptable. (Also, most of those V-shaped formations you have seen in old WWII movies were designed to allow on-board gunners to protect the other aircraft in the formation from enemy fighter aircraft.)
As it turns out, flying the tankers nearly line abreast, about 85 degrees, allows the SR-71 pilot to see the other one or two tankers. It gives him the confidence that he is not on a collision course with a tanker.
The tankers are separated vertically by 500 feet. There is a half mile between them, from wingtip to wingtip.
In addition, there was a maneuver called the crossover. Once refueling was completed, the lead aircraft would lose airspeed by climbing 1,000 feet (or 1500 feet in a 3-ship) and slide behind and over the right. There they would assume the new 85 degree and 500′ above position and the previous number 2 would become lead.