I received this rather exasperated response from Annika Thunborg, spokesperson at CTBTO:
there was no professional connection or areas of interaction whatsoever between the two persons who died eleven months apart in two completely different buildings at the UN in Vienna at which about 7000 people in total work and visit every day. For questions about the other death, please contact the IAEA.
So while I still don’t have the name of the other dead UN scientist, that seems at least to dispel the wide internet reports that they both jumped down the same stiarwell. Indeed I am left not sure whether the first IAEA scientist fell down a stairwell or died in another way. But plainly there remain a great many unanswered questions here.