Volume control for both in the same time is no problem because I just turn the monitors off when I use headphones. I asked this because wasn't sure if the headphones needed to have also the cable connected from iNano minijack (in) to my soundcard (out) which isn't possible in my case since there is no minijack out in my soundcard.
So if it works, this solves my problem!
Thank you very much for the info!