ah you wanted to run them at the same time? i just thought you wanted to use them independently...
with usb you cant coz it automaticaly mutes any other output. but if you were using 35mm jacks then you can turn off mute speakers when front jack is pluged it. sorry i got confused...
you can only select 1 output at a time... meaning that if you have realtech. you can use the audio panel to turn it on and all the jacks will be live audio so you can plug in multiple devices and get output from them all. but usb will use a different sound engine thus muting the realtech...
the problem lies with windows itself it only allows 1 output device at a time to be use. but like i said if you have a sound card that has more than 2 35mm jacks you can set them independently form each other... you can set multiple front outputs by setting them as line out and you can plug in 2 mics and set them as input... your still only using 1 input device so windows wont notice. but like i said all usb inputs are classed as there own devices so will cancel out any other devices used.
if your getting lost this may help...
you have 2 headsets each with mics attached. meaning you have a total of 4 35mmjacks. 2 for mic and 2 for output audio.
you have onboard/sound card.. that has a minimum 5.1 output ie 4 audio jack females. go into windows audio and select that device for output... in this case realtech audio. just select it and make it the default audio device.
click apply and go to the recording tab...
again select the realtech as the default recording device , click apply and quit.
now go to the task bar and select realtech panel
now plug in 1 audio out jack. you should see a popup asking what kind of output you want to set it for. select front out... this will give stereo out for the first headset... now plugin the other mic jack into the pink and again a popup will appear and select mic input.
next do the exact same for the other headset using the other 2 jacks selecting audio front out and mic input respectively...
you should now have 2 headsets that have the same audio output and both headsets should allow you to use each mic independently...but at the same time, as your only using 1 input device over all.
if you now plug in a usb headset it would be treat as another output device not as a headset but a sound card...
hope that helps clear it up.