When one thinks of "partnership" commands, one may think of the Duke of Marlborough and Prince Eugene. Perhaps Hindenburg and Ludendorf in World War I Or Robert E. Lee and "Stonewall" Jackson. In each case, the senior commander was older than the junior commander.
I can think of only one military "partnership" where the senior commander, Villars, was younger than the junior commander, Boufflers. http://www.spanishsuccession.nl/boufflers.html These were the men that opposed Marlborough and Eugene at Malplaquet. In civilian life, such a "dual command" might consist of Warren Buffett (aged early 80s), and Charlie Munger (aged late 80s), Chairman and Vice-Chairman of Berkshire Hathaway.
I'm not talking about situations with a senior commander and a "random" (but older) junior commander, in the ranks, but ones where the junior is "next to" the senior and is effectively the senior's "alter ego" or even "Mini Me." To take the example's I've given, Churchill described Marlborough and Eugene as "one soul in two bodies." Lee referred to Jackson as "my right hand." Munger has been referred to as Buffett's "Doppelganger." Were there many such situations in history?