There isn't an easy way of doing this and I'm not sure it's even possible; getting the speakers and the headphones to work at the same time is a big enough of a challenge!
How exactly are your speakers and headphones plugged in? One in front audio out and one in back audio out...?
I could have both speakers and headphones to work at the same time by plugging in one to the back and one up front - Realtek onboard.
But having both playing different tracks is not something i've tried or heard of.
Why don't you try left and right pan and using just your headphones? I do that easily with Acoustica Mixcraft. You could get different sounds coming from both left and right.
But if you're still looking to do what you want to do, i have no idea. I'm just suggesting an alternative. Someone else might have an answer though.