Hello,
You could get a splitter like this...
or this...
However they can degrade the quality of the audio.
Plus, headphones and speakers individually have certain optimal volume levels which are needed to recieve loud enough sound. This will be difficult to achieve if you've plugged both headphones and stereo speakers into the PC at the same time, using a 3.5mm splitter.
...Plus, you'll risk blowing the drivers on your headphones in this configuration when, for example, you've got both plugged in, and you raise the volume on your PC for sound to come out of your stereo speakers louder. Not to mention, you'll probably hear a distorted hissing sound emitting from your headphones when using your stereo speakers, due to the high volume.
...Is it that you want to recieve audio in both the headphones and stereo speakers at the same time...or is it that you don't want to have to constantly unplug and plug in headphones and speakers when you need them?
My advice. Have either the stereo speakers or headphones plugged in, but not both, in any configuration you choose to use. It's just not practical nor conventional.
Sound card manufacturers expect consumers only to use only one medium for outputting sound, and this does make sense.
However, if you are desperate, I strongly suggest you invest in one of these, or similar:
...It's an
Audio Hub. I haven't seen many of these, apart from the one pictured above...but I'm sure there are plenty on the Internet.
It's basically a hub where you can connect various sound output and input devices into, and use an integrated switch to select an output. It doesn't split the audio, but switches from one output to another, meaning no audio degredation. You can get it from here:
http://www.zipstorage.com/muauhub.html and other places on the net if you search.
Here's another from Radio Shack:
http://www.radioshack.com/product/i...udio+switch&kw=audio+switch&parentPage=search