Question about drop rates

I apologize, this is probably a really dumb question. I've gotten into multiboxing recently and I want to increase my chances at getting rare mount drops (like from Onyxia, for example). If I am running with a group of 5 of my bots, do I have 5x the chance of getting the mount to drop? Or would my chances be better if I took each of those characters and took down the boss individually five different times?



And is it different with the Pandaria world bosses, where individual loot seems to be generated on a RNG basis?

Question about drop rates