Wouldn't this mean you'd break the copywrite - by putting on a public performance?

You would have to check you buddy had purchased there own copy prior to sharing you buddy buds.

Quite a nice idea I think and potentially useful for any mobile/silent disco attendees.. (do those things still happen? Haven't heard of any for a while)

...just buy and carry a 3.5mm to 2x3.5mm jack headphone splitter adapter and tell 'em to plug their 'phones in.

Rather that than this daft suggestion of a product.