I'm looking at using Arcanum of Rapidity to enchant my head and leg slot items. https://www.wowhead.com/classic/item=18329/arcanum-of-rapidity

It says "Does not stack with other enchantments for the selected equipment slot."

What exactly does this mean? As far as I know, we can't have 2 enchantments on the same item. Does this mean that it will only work on head or legs, but not both at the same time?