If you use together Mythical Wyrmbone Mask (100% Physical Damage converted to Lightning Damage to Forcewave) and Mythical Handguards of Justice (50% Physical Damage converted to Fire Damage to Forcewave) what does it mean to actual skill?
Physical damage conversion to 66% Lightning and 33% Fire Damage? Or some items have priority over another one?
Can we use many modifiers to same skill without problems at this moment?
Many modifiers to one skill can work, no limits afaik. Multiple forms of conversion on the other hand… well it follows the order as listed in the guide.
Item modifiers count as skill modifiers IIRC. So long as both conversion forms are from the modifiers (the two items you mentioned) you will have as you said, ~33% fire and ~66% lightning. Items have no priority over one another.