I have an ATI Radeon HD 5700 with 2 monitors.
In Windows 7 I can use both screens fine (I mean like having a single monitor).
I have also installed Kubuntu, but when I log in, the monitors are cloned.
What must I do in Kubuntu?
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you can install ati card driver and then the ati catalyst control center will install in it under Display Manager choose multi display and you will find it there
I can start you on your way, although this might need some more padding from someone with an ATI card.
You'll probably need the propriety (i.e. from the manufacturer, rather than open source) drivers installed. There should be an notification looking for your attention, but I'm not sure where it appears in Kubuntu. In lieu of that you can use:
This should get you up to speed with your drivers, if you aren't already.
For the dual monitor you may be able to use 'Monitors' in the 'Control Center' to make it dual screen (rather than cloned.) It is possible, like nvidia, that you'll have to use ATI's software, however. A quick google search has suggested you try this:
Not being sure, however, I'd recommend you back up your video configuration first; just to be safe:
The KDE Display and Monitor settings are: System Settings > Display and Monitor.
With the Size & Orientation module you could put the monitors/desktops side by side or on top of each other.
With the Multiple Monitors module you could set a single desktop spread across multiple monitors.
The older hardware can have limitations how the desktops can be positioned: http://kubuntuforums.net/forums/index.php?topic=3119450.0
Generic Ubuntu documentation