This is the relationship of all relationships. That's not to say it's the best or even among the better, usually. But Libras crave relationships, they tend to develop and mirror their own person in relation to a close special someone. At times, this will be the sweetest of all friendships, seductions and romances. At other times, one of the Libras can't believe how ambivalent, egotistic and shallow the partner is, while then suddenly showing the very same tendencies themselves. The combination may work if their analytical personalities harmonize well and give enough room for some craziness and loyal, unquestioning affection.
It's a very social combination, both in terms of mutual communication and in the usual sense - lots of friends, acquaintances, parties, meetings and the like. They are both diplomats and big charmers, so while the champagne is opened and poured, there will be sudden bursts of jealousy and accusation. But they are seldom lasting. Libra is essentially a mental sign, and not so emotional. They can react strongly and fall madly in love, but aren't normally deeply brooding people with the tormented kind of emotionality. Not the most stable, but a stimulating relationship.