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Somebody should make a standalone media player where you can load two songs up, they play in sync, and you hit a button to crossfade between them, so you can listen to stuff like Fire Emblem's two-part battle tracks or the soundtrack to the Flash game Continuity the way they function in game
Another thing I'd settle for is a dual-playlist player where you can fill out one list with "quiet" tracks and another list with "action" tracks, and just hit a hotkey (or wire up a game script mod?) to shuffle to a track in the opposite list, as the mood dictates.