I do it a little different. I have a couple desktops, one set of monitor speakers, and a couple headphones now and then. Little different scenario, but I think my solution would work, though it's not a Windows 7 solution per se.
I use a small mixing board.
In your case, take your sound output, run to the mixing board. The mixing board should have a number of options for sound output. You could run the stereo mix to your main computer speakers (if you roll that way), and plug any number of other devices (your headphones) into the headphone out, control room out, stereo sends, etc...
For what it's worth, this is the one I use. May be more mixer than you need, but there's room to grow, and they're not too terribly expensive. A smaller sibling here is tiny, but actually still has 3 stereo outputs. In any case, there are a number of options from various vendors that can accomplish the basic mixing you're trying to do.
Be careful though, you can get really used to the control :)