Two fire signs together are pure intensity: passion, initiative, zero boredom. You understand each other's urgency to live. The obvious risk: two flames compete for the same oxygen — learning to yield center stage is what makes this last.
One holds steady (fixed), the other flows (mutable): the fixed one gives the mutable roots, and the mutable teaches the fixed that changing isn't losing. Complementary — as long as neither caricatures the other as 'stubborn' or 'flaky'.