Hmm. This example is somewhat confuse.
I understand that, if Prismatic changes the element to poison, the poison extra damage/elemental clicks kick in.
But as far as I know,if I have poison OH and poison head, unless I use a OH skill (say… traps), or the OH has explicit +weapon or +weapon% damage (adds to MH), then the poison damage is not dealt. We are not taking into account any damage boost from the OH (assume no other affixes, such as crit chance/attack speed, say it’s just empty sockets).
Given that case, Prismatic would not possibly change the element of the OH
By this implication either I’m wrong somewhere, or you have implicit either +weapon damage, or using OH skill.
Actually, on second thought, even many other affixes should not add Poison damage when it’s a MH skill, such as atk speed and crit chance. It’s just hitting faster or harder with whatever we’re using, in the case of a MH skill, ice.
The part about converting elemental crit into another element’s elemental crit is clear though. I just never saw any of them yet except High Voltage in some Legendary Axe.
Also, I thought Prismatic, if triggered, would forcibly change the element type to the one element the target is weakest to, but I guess I’m wrong there too. That was the main reason I was considering to go for Prismatic.