OK, this is just an idea I came up with in the past 15 minutes and isn't really well thought out but oh well.
Dual Character Abilities are abilities that require two characters to do. (Duh.) They combine two moves and make them into one super combo move thing. For example, a Shaman could use a Tornado like move and a Mage could link up with the Shaman and throw a couple fire balls in there and make a fire tornado type of thing. I don't know how Blizz could implement this but its just a random thought I had and wanted to tell someone :P