you can double up on some affixes, but only if there is a matching Myth Stone with the same affix.
if you have a Crystal +45% Cool Down on an Item, you could then put one +10% Cool Down Myth Stone on the same Item. if the Myth Stone is on the Item, you can’t roll the affix at all, so I would have the +10% Cool Down, but couldn’t roll +15% Epic Cool Down or +45% Crystal Cool Down. if you take the Myth Stone off, then you can roll the affix and then put the Myth Stone back on the Item.
there are 2 versions of Elemental Critical. Epic only gives Elemental Crit for a Weapon of the same Element. the Legend version can give Elemental Crit to Weapons with different Elements.
Epic: Poison MH Weapon & Ice OH Weapon. Epic Toxic would give your MH an Elemental Crit, but not your OH. if using Prismatic Talent or Ascendant Set, all Elements of the MH would have a chance to Elemental Crit their respective Elemental Crits, but the OH would have no Elemental Crits for any of the Elements.
Legend: this would allow the weapons used above to have Toxic for the MH & Freeze for the OH. using Prismatic Talent or Ascendant Set, all Elements for either Weapon has a chance to Elemental Crit their respective Elements.
Epic Elemental Critical is specific to the weapon.
Elemental Critical Damage is specific to the Element. if you have Blistering on your Fire Chest with Prismatic, whenever your Poison or Ice Weapon cause a Fire Attack, it would cause Blistering if it also gets an Elemental Crit.
Elemental Damage % or + must be the same as one or both Weapons, or you don’t get the benefit of the extra damage. so if there was a +100% Fire on the Item with Blistering, the Poison & Ice weapons wouldn’t get the +100% Fire Damage, but with Prismatic, could still get the Blistering. now if you changed that Chest Item to Poison for +100% Poison and +100% Blight, your Poison MH would get the benefit of both, but your Ice OH would only get the benefit of Blight when using Prismatic and getting Poison Attacks.
except for Crushing Blow and Crushing Flames, anything that does damage has a chance to do Critical Damage. with high Critical Hit Chance, you can sort of stack your damage.
if your Fire Attack does a Crit Hit, your Fire DoT damage would be based on the Crit damage. then your Fire DoT also has a chance to do a Crit Hit. Immolate is based on your Base Fire DoT damage, I believe, but Immolate can also cause a Crit Hit. Blistering is based on either your base Fire or Immolate damage, I think, to cause Bleed Damage, which also has a chance to have a Crit Hit.