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 initiates (cardinal), the other sustains (fixed): done well, it's the formula for projects that start AND finish. Done badly, one pushes change and the other digs in. Negotiation is this bond's muscle.