Flame totem questions

Why do people use dual wands for dual flame totems? And what gems do they put in the wands?

Two wands with a combined total of 69% faster projectiles is equivalent to a lvl 20 faster projectiles gem. You can then link something else like iron will or added chaos damage in place of faster projectiles on your chest piece for added damage. I've ran phase run + increased duration and lightning warp + faster casting + less duration in my wands.