One set of arms are jacket-sleeve arms. The other arms are dictated by whatever's under the jacket (i.e. bare arms for Dagobah/Bespin Luke, white longsleeve shirt for Bespin Han, etc.)
I only follow SW stuff, really, so I don't know how 21st Century's done it, but even Hasbro's done it, in a way. Anyone remember the Dodonna and Antilles figures from two years ago? Dodonna was wearing the long white coat, and Antilles was wearing the jacket? The arms were easily pop-out-able on both, so if you wanted to give Dodonna a jacket to wear, like during the Battle of Yavin, you could easily do so.
You'd just pop out Antilles' arms, slip off his jacket (which was then basically a vest). Then you'd do the same to Dodonna's arms and coat, and put Antilles' jacket/vest on Dodonna, and pop Antilles' arms in. Easiest custom ever--so easy that I don't know if it could even be considered a custom. It almost seemed like Hasbro designed them to be swappable like that. Maybe they did.
The only real drawback to that would possibly be the articulation in the shoulders--I'd imagine it would have to be limited to swivel style, instead of the preferred ball-joint style. But that's kind of a small trade-off to make. I think it'd be cool to get a figure like that--it's almost like two figures in one, yeah?