Ceiling light fixture held up only by the pressure of two screws?
Yes, this is very normal thing. The end of the thumb screw engages a lip on the glass cover or globe. It is more common that there are three such screws. Here is a side view cross section to show approximately what it looks like.



To remove the globe or glass cover you simply use the thumb head to back out the screws till the lip on the glass part can drop down. 

Note that on some fixtures there may even be only one thumb screw with one or two fixed pins or ledges on the metal fixture on the far side. This type you back out the thumb screw and then have to tip down the globe till it disengages from the fixed points on the other side.