The 390X will consume about 240W, this means it's possible for my ss 520w psu can handle 2 390X?
The 520W cannot handle two 390X unless you undervolt and under clock the R9 390X to consume about 200-220W.
is not wise to run a psu 100% of his loat 24/7 ... more over it depends on the amper you ahve on the 12v rail.... a psu has many rails (3 5 and 12 volts...
ie : i have an enermax revolution 87 850w
it is rated 840w at 12v but can give up to 970w if u consider also 5v and 3v
if your psu is rated at 12v 520w then u can plug 2 cards but make sure you undervolt em a lil and make sure you have a quality psu