It doesn't need to power the Z706, it has its own built in amplifier. All the U7 will do is control the output.
As for simultaneously...why? Unless the games can put all comms (even in-game with AI characters and game advisers like the geisha or lady that pops up on the upper left on Total War) on the headphones and everything else around you so it sounds like you're on a comms system amid all the noise of an actual battlefield, I don't see why you'd want that.
In any case, it's not the power that will be a problem considering the Mobius is primarily a portable with its own amp built in (I think it bypasses the built in amp when hooked up), but that chances are you can only choose one output. On a DAC-HPamp for example if you have active monitors or a power amp driving speakers on the preamp output, that output gets cut when a headphone is inserted into the headphone jack, and for those that have output selectors like the AudioGD, you can only flick the switch to do one but not both.
EDIT: I just looked up the Mobius again and it looks like it's not just a wireless headphone, it appears to have its own DSP. Are you sure you don't want to just use its DSP and hook up the speakers to the motherboard's 5.1 output?
U7 will be enough for the Moebius. I'd worry about power if you get an LCD-x or HE560 later.
Didn't have a problem with my U3 and any player program.