The FranXX’s are based on Klaxosaur biology. The important feature therein being their binary existance.
Every Klaxosaur is a Jian: The main body (Female) is paired with a core (male). The female is supplied and directed by the core, while the male is empowered and protected by the body.
Female pilots emulate the body of the klaxosaur by diving into the machine’s mind. The machine then feeds them magma energy, which is activated by their negative pulse. Along with, the male pilot sends a command signal through his positive pulse. All things going well, you have a reasonable emulation of klaxosaur biology.
The stamen can’t activate alone because the female is necessary to activate the magma energy.
The female SHOULDN’T activate alone, because she’s then sharing a body with a brainwashed klaxosaur… Which doesn’t sense her partner, and gets mighty pissed about it: It encroaches on the Pistil and adapts the FranXX to a more familiar form. The Pistil maintains control by “being” this pissed off Klaxo-female, but it has some worrying implications for her sanity and humanity.