Symmetry.
Al likes motor oil, complicated tools, strange books about energy, mass, speed and other laws. He likes to sit by the window, feeling the cool air and the hot sun, working a small sweat under the strain of notes and papers. He sketches blueprints for things that make no sense to anyone but him and sighs contently every time he reaches another complex jumble of lines and numbers.
Al likes dusty old libraries and thick history books about alchemists and their craft. He likes to read outside, where there's a chance to find something furry and nice to keep him company while he contemplates how to break the world in two. He always has food on him, willing to share it with the newest pet he's adopted, and smiles happily when he's rewarded with a purring noise.
Al and Al rarely walk together, rarely talk about anything beyond the climate and how Ed's going to get himself killed with his newest scheme. Al and Al rarely sit down and drink tea while discussing the finer points of their arts and how to join them efficiently.
But when they do, it's mesmerizing to see the identical faces lit up with amusement or mischievousness, to observe the way their hands wave around in the same delicate arc, to catch their fingers lacing with one another, measuring the disturbing similarities.
Al buries his nose in his notes to reach the sky and Al tries to procure a decent home to any and all meowing creatures that come across him, but at the end of the day, when they're too tired to put up a wall of differences, similitudes overcome those blind enough to not see.