Its pushing it a bit close for an unknown quality PSU.
It's one of the best PSUs(fsp stands for Fortron Source Power which is the well known brand Fortron) - what do you mean by unknown

? Just look at the awards list on the link that I've posted.
My point is if the PSU is 1200W (80 plus gold and stable characteristics) - is it ok to handle 2*590W=1180W(which is supposed to be the power of the two antminers S5) at the regular hash rate without much overcloking ?
People have reported running two non-OCed S5's on a 1200W PSU so you might be fine, though it is pushing it a little close. You'll be running it at 90% or so load 24/7, so just avoid any overclocking.