The headphone jack includes a physical disconnect that diverts the signal from the internal speakers to the external jack. Therefore when something's plugged in there, the internal speakers are non-functional.
However, it is possible to use multiple speakers if you're using USB speakers, or speakers connected via some other manner, although support for this is application-dependent (most applications play through the output device selected in System Preferences -> Sound, but some allow you to override this).
In addition to that there's the option to play system alerts and sound effects through a specific device while the default audio output is a different device. Coupling this with an application that allows its own overrides and you can get some interesting combinations. Note, though, that each application is typically limited to a single output only unless, again, it provides an override (e.g. iTunes has the ability to play through multiple output devices at once).
The headphone jack includes a physical disconnect that diverts the signal from the internal speakers to the external jack. Therefore when something's plugged in there, the internal speakers are non-functional.
However, it is possible to use multiple speakers if you're using USB speakers, or speakers connected via some other manner, although support for this is application-dependent (most applications play through the output device selected in System Preferences -> Sound, but some allow you to override this).
In addition to that there's the option to play system alerts and sound effects through a specific device while the default audio output is a different device. Coupling this with an application that allows its own overrides and you can get some interesting combinations. Note, though, that each application is typically limited to a single output only unless, again, it provides an override (e.g. iTunes has the ability to play through multiple output devices at once).
