I have to concur that this idea is one of the more ridiculous concepts I've heard in a long time. So, here's WHY NOT.

• Weight. The cases could easily hit 10 pounds each; a tandem with S&S couplers adds at least 10 pounds. You could be adding 20 pounds just in luggage. And with so much space in the cases, you will undoubtedly be tempted to load them up with unnecessary crap.
• Bulk. What are you going to do if you have to lug this contraption up or down a flight of steps? Oh, and have fun getting to the airport with two 62" cases, loaded with 35 pounds of bike and who knows how much gear.
• Airport Cost: With two huge heavy cases, you'll get nailed with luggage fees coming and going.
• Build Cost/Complexity. Building a coupled tandem is, to put it mildly, not easy -- if you want it to hold up, you're going to need experience as a framebuilder. You'll also have to come up with your own coupled design, because you can't buy S&S couplers direct -- only licensed framebuilders can retrofit S&S couplers. And if you buy a new coupled tandem, that can easily set you back $4000+.
• Drag. 62" linear cases will be bigger than the biggest panniers, and will slow you to a crawl in any headwind.
• Design. Tandems are designed for a second rider; putting a pair of massive cases on the side is, undoubted, out of spec.

Oh, and I'd hope it would be obvious that if you don't build this bike properly, you could wind up in a crumpled heap buried under half a tandem and a pair of huge suitcases, and chewing on tasty asphalt.

Better solutions abound, as already evidenced over and over again in this thread.