Like most spells, its just hard-coded into the game. Try taking the "Special" one in the units tab under Neutral Hostile out, and see if it does that too, I doubt it does, but you never know. If it does work, use a spell like Archimonde's Dark Portal and change the values around. Thats about all you can do to get a permanent one.
If it doesnt have the attack-split thing, you'll probably have to trigger it to do so, but I've got my money on the split thing happening.