Two spears, for real? And here I thought I was going with an oft unused weapon. Such is life. 

Is Serpent's Fang super useful, though? If you've already stabbed them with your spear, you've kinda got them in a bad position anyway.

I suppose you could wipe the poison onto something else, like a blow dart. I'd say an arrow, but that would have the same deal as the spear itself: it kinda kills well enough without the poison. 

Unless we're talking larger scale creatures. On the subject of which: is there a size at which a dose of the poison will be insufficient to kill a being?