Magnerobo Ga-Keen (1976)

Dr. Kazuki, who perceived of an invasion of Earth by the Izaru people builds a robot based on the science of magnetism and sphere joint theory. Undergoing a dangerous augmentation process, Dr. Kazuki's daughter Mai becomes the pilot of "Mighty" or "Magnetman Minus". Takeshi Houjou becomes the pilot of "Puraiza" or "Magnetman Plus". The pilots would hold each other and then physically trasform their joint bodies in a metallic plate locking itself on the Gakeen (Short for "Gathering Keen") robot's frame thus enabling the super robot to move and fight. This sequence was notable, to an adult's eye, for its almost-sexual connotation. (Source: Wikipedia)