This is either a special product made for a very special person (with four ears) or a decidedly old-school way of getting around file-sharing laws. Either way, we wouldn't count on a huge market for this double headset.
Akihabara News says SKPAD's latest invention can be used with any device that has a standard headphone jack, though we're not sure about quality. And because it seems to work like a splitter or an adapter for multiple earphones, we're also not sure about the $37 price.
Something like this might be useful on a plane or in the backseat of the car if your kids are fighting over an older DVD player that has only one jack. But if that's the case, you probably have problems a lot bigger to deal with.