You need Mario / Sonic instead of Mario / Bowser. Seriously.
They have shared games, making them eligible, and their rivalry is legendary (though that's more because of the time they didn't share games).
Edit: Also, Leon / Krauser >>>>>> Leon / Ada
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"I am SO gonna kick your ass!" - "No, no, no, no, no. I'm gonna kick YOUR ass!"
Nominate Yuri Lowell / Flynn Scifo