To make a hybrid Essence/Mentalism Spell Adder (or Multiplier), make one each of Essence and Mentalism. Then get a friend and stand at opposite sides of a large room or field, each holding one of the Adders. Then run at each other as fast as you can, holding out the Adders. If you do it right, when the Adders collide, they will combine into a hybrid Spell Adder.
Note that this is not an official ruling, but this is the fun way to do it.