BuDaThe x7 ''acts as an external USB DAC and a powerful audio amplifier with low-latency Bluetooth® connectivity and Dolby Digital decoder ''. The Katana has a USB DAC and amplifier, bluetooth capability and and a Dolby decoder. The x7 will be doing nothing for you when using the Katana, except adding an unnecessary extra layer of signal processing, plus you need to directly connect your PC to the Katana via USB to get the 7.1 simulation.
BuDaYou shouldn't have any problem having them connected simultaneously via USB, if that's the answer you're looking for. I've got 2 sound devices on my PC, one running my normal audio and the other running communications. I've never had trouble with it.
BuDaYou're welcome. Just a quick heads up, once you have them both connected you'll begin getting intimately acquainted with the Sound menu in your Operating System. :)
BuDaWhat I mean is there would be no use for your x7 if you had and were using the Katana. The Katana does everything the x7 does. The only thing that might be better about the x7 is the quality of headphone output, but the Katana does have a headphone output as well. Going through the x7 to the Katana will not improve the sound coming out of the Katana, it would probably make it worse because it would reduce the full performance potential of the Katana.
As Mofo said though you could have both units connected to your PC if you wanted to switch between systems.