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General opinion seems to be that a union of French and Spanish Bourbon Kingdoms is ASB unless you at least gank the UK, and even then would be difficult to maintain. How about a more modest union of Bourbons crowns - France and Bourbon Italy (1735-1860, with some revolutionary interruptions)? This is of course somewhat complicated by Sicily and Naples being two kingdoms loosely united under one crown before the Napoleonic invasion, but I am sure something can be figured out.