Thanks. What I meant by "stack" was this:
Water creatures get stuck in a fire vortex, and lose 1 HP per turn for 3 turns. If a water creature suffers this effect twice in a row, they will only lose 4 HP over 4 turns instead of 6 HP over 6 turns.
I think that the elements should contrast each other more physically as well as abstractly in some way. Holy light can harm death and darkness, but other than this the game doesn't really involve direct opposition between elements. I understand that it would be quite situational, but I think that if it is just a small boost, it would be interesting.