I'm in the same boat, Sena. Just got my second r9 270x yesterday and I've spent the last 8 hours trying to figure out how to get it to work. Fresh Install of OS - Installed drivers with one GPU. Installed second GPU - Brought up CCC and it will not allow me to check the crossfire box. Unfortunately it sounds like I'm going to be buying another motherboard to just make this work.
I fixed it eventually. What i did was: first, uninstall all your cards drivers, second, take your both cards out and run/reinstall their drivers just one at a time, then, switch them (from their previous positions in the PCI slots).