You can get switching TRS jacks, so it'll (for example) play out through the speakers in the back until you plug a headphone in through the front. Otherwise, you can just have a switch, so you have 1 or more outputs that you plug in, and you select which of them gets the output based on the switch.
I plan on doing this with a LCD-unio eventually.
As far as having multiple outputs running at once, one major problem you'll have is that they'll be different loudnesses (unless you have two of the same model headphone). There are ways of circumventing this, though. (I think you can just use another potentiometer)
Here is an example of a switching-type jack.http://www.neutrik.com/us/en/audio/210_ ... etail.aspx