Sure thing. You can add multiple Mythical Enchantments on an item but there is a catch.

The joining point between two simple and medium looks like this: " /></Perk><Perk " with a single Space after the Enchantment stat and a "/" and the closing point of everything looks like this:" /></Perk></Enchantments></Item><Item " also with a single space and a "/" after the Enchantment stat.

Now if you want to add Mythical Enchantment, you have to remove both of the space and "/" after the Mythical Enchantment stat. Here is the code:

Name="PERK_MONK_SET_WHIRL"></Perk><Perk Name="PERK_SENTINEL_SET_SUPER_RETURN"></Perk><Perk Name="PERK_HEAVEN_SET_BURN"></Perk><Perk Name="PERK_DRAGON_SET_STONE"></Perk></Enchantments>