It is enough to render the desktop. My notebook has 2 graphic cards. The better one is deactivated while I am on battery (this make it also possible that the battery lasts approximately 6 hours in normal usage), while I can activate the second one if I need the power. It is one of the first subnotebooks which can combine both, mobility and performance (even in 3d applications like games). Also most CPUs have today an integrated GPU.
But if you want to do more than just accelerating the desktop or simple 2d stuff, then you should really use another GPU! If you have performance problems in teeworlds (because it needs a high framerate), then try to compile the actual git hub version, there you doesnt need a high framerate to be able to play teeworlds well (but a little bit more cpu power).
The face of terrorism is called United States of America!