How to get headphones and speakers working at the same time?

You need to disable "Front panel jack detection". This will make the front and rear jacks play the same stream.

Source: http://www.tomshardware.com/forum/243915-49-realtek-audio-manager-sound-signal-simultaneously-front-rear

I solved the problem with different output to the front panel on my ASUS P5P43TD.

All you need to do is to go to Onboard Devices Configuration in BIOS settings and change Front Panel Type from HD Audio to AC97

After restarting you have now Tab called HD Audio 2nd output in Realtek Audio Manager (but keep Speakers tab as default output).

If you can't see that Tab, go to Device Advanced Settings and change it to Make Front and Rear output devices playback two different audio streams simultaneously. If you choose the other option in Advanced, you will have only one stream but from both outputs - front and rear.

And now you have multiple choice for output in popular applications such as audoi- and video-players, where you can change output to Speakers or to HD Audio 2nd output.

As a result my Foobar2000 (audio) and MPC (video) are playing onto different outputs as I'm typing this post.

For some games you can choose the output too (Black Ops) in game Sound options but in some of them you cannot (MW II) BTW if you don't set anything as Default in Realtek Audio Manager you won't have any sound in MW II and some other games.

You could always buy a headphone splitter for a few $$ or ££ or €€