headphones and 2.0/2.1 speakers share the same 3.5mm jack. sometimes you do have a front panel jack as well but this completely overrides any rear ports. no idea on how you plan on using both without jumping through all sorts of hoops.
finally if you did have them connected somehow then the audio sources would have to be directed to different outputs via some sort of software.
i'm not sure but usb headsets may be easier to workaround but you will probably still need software which might even be included if you are lucky.
in any case... isnt using the speakers negating the whole point of wearing headphones?
This really depends on your sound card wether or not this is possible. Some sound cards (such as with Realtek) have a built in audio manager. With Realtek there is an option under Advanced Options in the Realtek HD Audio Manager to make Front and Rear output devices playback two different audio streams at the same time.
If your sound card is a Realtek card or supports similar options, after you have selected the option to split the Front and Rear output devices you would need to configure the Output device in your programs to select which of the two output devices you would like audio to go to.