I only use open source drivers.
sudo apt-get install xserver-xorg-video-radeon
Tell source to load readeo driver, create the file /etc/X11/xorg.conf.d/20-radeon.conf
with the following content:
Section "Device"
Identifier "Radeon"
Driver "radeon"
Option "TearFree" "on"
EndSection
sudo apt-get install xserver-xorg-video-nouveau
To tell Xorg to load nouveau, create the file /etc/X11/xorg.conf.d/20-nouveau.conf
with the following content:
Section "Device"
Identifier "Nvidia card"
Driver "nouveau"
EndSection
Uninstall the respective driver and install the other, the Xorg config file is the key here. Otherwise, Xorg cannot know which driver to load.