I'm sure this example of a double cathedral city is cheating, but what about Autun? I believe that the existing 12th cent church of St Lazare, which was in fact the canons' church, was used as the bishop's seat for periods in the later Middle Ages, while the 'proper' cathedral, St-Nazaire, just across the road (which now only survives in fragments incorporated into later buildings, and a recently excavated cloister), was under construction. Andrew Jotischky Department of History Lancaster University