When I insert both, Xserver (donno what is this) wouldnt start, xinit/xstart wouldn't work,
What do you mean by "wouldn't start"? What's the error message?
Other info: my psu is 750w bronze
That won't handle both cards at full load,
this review says 365W per card...
My mobo is F2A88X-D3H, it has 2 pciex16 (one x16 one x8)
The slots are railed together, meaning that when you plug something into the x8 slot, the x16 slot gets throttled down to x8. Your problem might have something to do with that (the cards don't like x8?).
Also, do you have the cards bridged together or no?