Oh, the dynamic duo.
The archetypal fantasy novel is a cast of thousands; others succeed by focusing on a single character or thread. As satisfying as the lone hero(ine)'s adventures can be, however, there's added dimension when they have a counterpart, ally, friend or even adversary to provide another perspective. Get into three main characters and you start to have exponentially more combinations of plot and interaction, but two allows greater variety without diffusing focus. It's also a great opportunity for compare and contrast.
Even if the characters have a common goal, they come at it from different angles and seek different rewards from its resolution. Or two characters may mean two separate plotlines, dovetailing in location, antagonists, events, and/or origins, but not always intertwined.
From a practical standpoint, it gives the writer an excuse to have the characters talk to each other - or talk around each other - rather than weighting down the narrative with lengthy internal monologue. Even when one (or both) aren't the sharing type, characters convey a lot of information in what and how they choose to evade.
As a writer, I'm a fan of the duo. Flow shares roughly equal scene-time between Kit and Chailyn, though it is primarily Kit's story. Scylla and Charybdis is definitely Anaea's sole story, but throughout, she always has a traveling companion to lean on.
And the character I discussed in my Song Styles post yesterday? Well, confessedly, the only other details I have right now for that potential novel is a rough idea of her traveling companion, an introverted dreamshaper ... but the collision of those two people is enough to suggest a rich stew of possibilities. For me, it's a tantalizing spark that could some day become a novel.