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[NASA caption] Launch personnel prepare to seal hatches on Gemini XII spacecraft in the White Room atop Cape Kennedy’s Launch Complex 19. The space pilots were launched at 3:46 p.m. EST on their four-day earth orbital mission, the last in the National Aeronautics and Space Administration’s gemini program. An Atlas launch vehicle, topped with an Agena target satellite, lifted off from adjacent Complex 14 an hour and a half before Lovell and Aldrin were launched, will rendezvous and dock their spacecraft with the Agena, perform a number of experiments and Aldrin will conduct three extravehicular activities.

“As was the case on the previous four missions, the Gemini XII flight plan called for rendezvous and docking with a target vehicle. But, according to Dr. George Mueller, NASA’s associate administrator for Manned Spaceflight, mastering what NASA called an extravehicular activity (EVA) or spacewalk would be crucial in proving the agency was ready to move ahead with Apollo and achieving the goal of landing a man on the Moon before the end of the decade.
‘I feel that we must devote the last EVA period in the Gemini Program to a basic investigation of EVA fundamentals,’ he said.
To take on the challenges of this crucial flight, NASA assigned a veteran of the longest spaceflight to date and the astronaut who helped ‘write the book’ on orbital rendezvous.
The Command Pilot was Jim Lovell who served on the 14-day Gemini VII mission in December 1965. Flying with Lovell was U.S. Air Force pilot Buzz Aldrin, the first astronaut to have earned a doctorate. In 1963, he was awarded a doctorate in astronautics from the Massachusetts Institute of Technology. His graduate thesis was ‘line-of-sight guidance techniques for manned orbital rendezvous’” (https://www.nasa.gov/feature/gemini-xii-crew-masters-the-challenges-of-spacewalks).

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“To make lunar EVAs possible, spacewalking during Gemini flights was a crucial learning experience. Ed White’s spacewalk on Gemini IV made it look easy. But the experiences of Gene Cernan, Mike Collins and Dick Gordon on three later missions demonstrated a new approach was needed for both training and performing spacewalks. Through Gemini XI, EVA training focused on use of the KC-135 aircraft flying parabolas. During the dives, astronauts experienced up to 30 seconds of weightlessness. But this was followed by the aircraft climbing and the astronauts having a period of rest. Consequently, spacewalkers in training were not facing the types of continued strenuous work and fatigue experienced by Cernan, Collins and Gordon. Dr. Robert Gilruth, director of the Manned Spacecraft Center in Houston, ordered a new approach. ‘I have given a great deal of thought recently to the subject of how best to simulate and train for EVAs,’ Gilruth said in memo to Deke Slayton, director of Flight Crew Operations.
‘Both zero ‘g’ trajectories in the KC-135 and underwater simulations should have a definite place in our training programs.’
The alternate approach uses a large pool of water for ‘neutral buoyancy.’ In this method special weights are added to the astronaut’s spacesuit creating buoyancy to offset gravity so the astronaut neither rises nor sinks. Aldrin spent several sessions of more than two hours each working with a Gemini mockup in the pool at the Environmental Research Associates facility near Baltimore, Maryland. Additional hand holds and the underwater training were keys to adequately preparing for the challenges of spacewalks. This approach became so successful, underwater training has become the primary spacewalk training method used by the United States, Russia and China” (https://www.nasa.gov/feature/gemini-xii-crew-masters-the-challenges-of-spacewalks).

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The two vehicles were actually launched 90 minutes apart from Cape Kennedy.

Gemini XII spacecraft atop its Titan launch vehicle, carrying astronauts James Lovell, Command Pilot, and Edwin Aldrin, Pilot, was launched from the Kennedy Space Center’s Launch Complex 19 at 3:46 p.m. (EST), Nov. 11, 1966.
An Agena Target Docking Vehicle atop its Atlas launch vehicle was launched from the Kennedy Space Center’s Launch Complex 14 at 2:08 p.m. (EST), Nov. 11, 1966.
The Agena served as a rendezvous and docking vehicle for the Gemini XII spacecraft.