Couldn't tell you specific statistical #'s but for one example, killed Xuekao the Deceptive 35 times in a row solo in nm with no daggers. First time in hm got the daggers, 15 more hm kills resulted in 3 more sets of daggers.
The only bosses I've ever seen drop more than one green at a time would be the darkness at the end of ToPk. In hm they will drop 2-3 greens per boss everytime.
Yeah for example I kill Kephet in NM and sometimes it drops 2 greens. Also in Sorrows Furnace NM, sometimes 2 greens in NM. Then do something in factions/nightfall/eotn and never get a green in NM. Seems weird?
From my experience there's no difference in greens drop rates in HM and NM.
And a Sorrow's Furnace boss can drop even all 3 greens at once (I've seen that myself and screenshoted), but chance for that happening is approx 0.1% - chances for every green to drop are calculated separately and most regular bosses have a green droprate of ~10-11%