1444. My Ally advantage and Arch Enemy flaw can't both represent the same person.
Warhammer 40,000 example: Leman Russ and Lion El'Johnson hated each other's guts for quite some time after their first encounter, in which the two primarchs ended up fighting each other over an honor issue, fueled by their vastlydifferent personalities, but over time, they became accustomed to each other's personalities and eventually became very close friends. Their rivalries remained, however, so this is the kind of friendship that evolved. Their successors sadly only remember the rivalry part of their relationship, however.
The Dungeons & Dragons 3.5 iconics Jozan and Lidda straddle the line between this and With Friends Like These.... She gets irritated by his pious and saintly straitlaced Good Shepherd ways, and he gets irritated by her unrepentant Loveable Rogue ways and love of constant teasing, to the point of near-constant Snark To Snark/Passive Aggressive Combat. But they'll still risk their lives for each other, and at one point Jozan admits to Regdar during a battle that he wouldn't mind having Lidda around to help. D'aww. No doubt they'd then start arguing again afterwards...