That launcher was unique as the booster stage worked the same way that a multistaged vehicle, excepted that the 2 engines were connected to the main stage tank, and the main stage engine nozzle was going through the booster stage. It was designed that way because in-flight ignition of a powerful second stage engine was judged risky, and called a "1 stage and a half" design. The Agena vehicle contributed to develop in-flight ignition technologies.
Also the animations don't feature the Vernier thrusters (those small engines on the side of the main stage), that allowed to control attitude of the rocket (no nozzles gimbal on big engines at that time, the russian R-7 was designed the same way and still uses Verniers to control attitude).
