This 'fic made its first appearance back during the summer; I kept meaning to recommend it but only now have gotten around to it. Catalyst
is an eleven-chapter plotfic which has the world-hopping protagonists of Tsubasa: Reservoir Chronicle
land in Amestris. In pursuit of yet another of Sakura's feathers, they encounter Ed and Al tracking down yet another rumor of the Philosopher's Stone. Well-thought out adventures ensue. The author is a dab hand at description and handles both sets of characters well. If you're familiar with both canons, have a look.