Now both my video adapters are turned on (according to lspci). I have found a fix, which turns off the ATI card.

I have some noob's questions now:
1. How is it possible that two videocards are working? Why don't they conflict? How do I know which card is performing the render?

2. could somebody explain me what does this code actually do (citation from that fix)?:

sudo cp lenovo_acpi.ko /lib/modules/`uname -r`/kernel/
sudo depmod
echo lenovo_acpi | sudo tee -a /etc/modules > /dev/null
And shall I be able to turn my ATI card on if I want?