Libra and Aquarius share the air element: you meet in ideas. Conversation never runs out — plans, theories, inside jokes. The risk is living in your heads: this bond does well to ground things and say what's felt, not just what's thought.
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.