Can i use two headphones on my x-fi music without Y splitter, headphones amp or additional hardware, so they receive same signal? I mean is there a way to use connector for e.g. rear speaker of the soundcard to receive the same signal to my 2nd headphones as for 1st?
Not knowing exactly how the Xtreme Music is set up, this may not work... but if you're determined not to use a splitter, than I can think of two options...
You might be able to do your front/rear out option. If you set the output to 4-channel and then CMSS-3D to Stereo Surround, that should replicate the front channel on the rear. I'm not sure how it would interact with 3D sound sources, and any playback software would need to be set to 2-channel output (which the X-Fi would then split), but it might give you the results you're after.
The other option would be to use the HDA connectors on the card, if it has them. I'm not entirely sure if the Xtreme Music has such a connector, but if it does than you would simply set the soundcard to 2-channel, then plug one set into the rear of the card and one set to the jack on the front of the case. Unless doing the latter automatically mutes the former, you should be getting the same output from both.