Alternatively, you can install Kubuntu on Ubuntu using Synaptic Package Manager.
Click System > Administration > Synaptic Package Manager on Ubuntu menu.
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Type Kubuntu inside the Quick search box, mark Kubuntu to install.
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You will be asked to mark additional required changes, just mark it.
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And click Apply.
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Click Apply again to confirm changes.
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Wait the package to be downloaded.
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You will be asked to select the default display manager, gdm? or kdm? It's the display manager of the login screen , any one is ok.
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Wait install and changes applied.
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Now, you can restart or log-out and log-in again to switch to KDE.
You can Select KDE Session by clicking Options on the lower-left of login screen.
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Now, your system have both Gnome and KDE installed, you can switch between in login screen.