The angle between your signs — the square (90°)
The square is the aspect of tension: you want the same thing at the same time, just not the same way. This bond never falls asleep. It asks you to pick your battles, and makes you both stronger when it holds.
Your elements — Air and Earth
Air thinks, Earth makes it real. One has the ideas, the other knows how to make them stand. Translation is often needed: you do not measure time the same way.
Your rhythms — cardinal and cardinal
Two cardinal signs: you both start things. Plenty of initiative — and the question of who leads.
Your rulers
Libra is ruled by Venus, Capricorn by Saturn. Desire and harmony, facing structure and time.