gurren-lagann
When Simon's Lagann mecha and Kamina's Gurren mecha combine by way of Lagaan drilling into Gurren's head, they form the titular Gunman of the Tengen Toppa Gurren Lagann series, known as Tengen Toppa Gurren Lagann.
Separated, neither Gunmen are particularly noteworthy or powerful, but together, their power rises drastically and they gain the ability to produce an endless amount of drills in different sizes. Using Lagann's innate ability to combine with other mecha, Gurren Lagann can also assimilate or combine with other Gunmen to enhance its own strength, often exponentially.