Alright, I think I get what you mean. You want the sound to keep playing while the spell is over the enemy, and then stop once it's off, right?
That's a little trickier than a normal sound effect, but it's doable. You should load the second effect into a specific channel first and set it to loop(make sure the effect actually sounds good when looping!), and play it while the spell overlaps the enemy. Add another condition that says when the spell DOESN'T overlap the enemy, stop playing that channel.
It's possible there's a simpler way to do it, but I did a similar thing in my own game using this and it works fine.