Dual monitor on OS driver - (Advanced Micro Devices, Inc. [AMD/ATI] Trinity [Radeon HD 7640G])

I have an issue with working in dual monitor mode.
Allways display only one of them or booth has a blank screen.
How to use xrandr for VGA-0 and LVDS to work a booth at the same time?
The monitor on VGA-0 has 1024x768 resolution and on LVDS 1366x768.