You also have the problem where you do not want to hook the two negative speaker outputs together. You want a resistor between the negative speaker output and the common negative hookup on the headphone. I believe most amplifiers would balk at shorting the negative speakers outputs together.
Single ended headphones have individual hot connections for the left and right sides but share a common negative hookup. You don't want to try hooking that to your speaker outputs. You may get away with that on some amps but I don't think you want to fry anything. Before you hook the grounds together make sure it won't damage anything.
Ed
We don't shush around here!
Life is analog...digital is just samples thereof
