Be careful though... most laptops are mono. doesn't the ring on a laptop provide power for the mic? That could be a problem if you were to send the + down the tip and ring.
anyway if you always know that you are plugging into a L/R jack, wiring the tip and ring together to feed both sides shouldn't be a problem. And it would be easy to make a jumper that floats the ring for those occasions when you know that you'll be using a mono input, or add a second output jack to your tin that floats the ring.
"I believe that entertainment can aspire to be art, and can become art, but if you set out to make art you're an idiot."
Show business: we're all here because we're not all there.
Resistance is not futile. It is voltage divided by current.