From what I have read, the poison arrow overrides the poison effect from the bow. Also if you shoot poison arrows very fast the following arrows over ride the effects of the previous arrow so you are not getting the full poison effect of each arrow, I did moder about a week ago using 140 Needle arrows when it was in the air and the iron mace when it was on the ground. Obsidian arrows should do well in the fight. The 140 arrows used up 3/4 of my bows durability so if you are going for complete bow fight, bring an extra bow.