In which two constellations, were the two gravitational wave events of 2016?
The data is indeed very uncertain. As for GW150914, LIGO posted this image of the probable area of the sky:



The most probable banana-shaped region stretches from Crux to the Large Magellanic cloud. Comparing the image with a constellation map, there are several small constellations that may have hosted the event. Those are Mensa, Chamaeleon, Musca and Crux.