Juxtaposition
They're two contrasting forces, often moving in the opposite of simultaneousness in regards to a shared emotional plane. Friend and enemy, weakness and strength. Yin and yang.
Ironically complimentary. (Isn't it always?)
The ability to know where to strike most damagingly, borne of an intimate awareness of the countering psyche, parallels the comprehension of just what to say to alleviate a sting.
For two people who cannot agree on a breakfast cereal, they amazingly function as one entity when their lives literally depend upon unified cooperation. Tacit and implicit, unison and understood, they're the epitome of partnerships.
And oxymoronic juxtapositions.