A cool and innovative concept, held back a bit by the unclear interface. I have played several rounds and still have trouble figuring out which nightmare does what. From what I can tell, bats and devils move faster, and the grim reaper hits everything in the square. But I also noticed bats and ghosts don't attack some enemies (skelletons for instance) and I don't know why. Also, most times one of the sibblings died it was because I spawned a nighmare he or she couldn't afford, and I couldn't see it.

Even within the hard-mode challenge, I think a few icons besides the spawn-buttons could have helped. One or Two arrows denoting their movement, a number or number of hourglasses besides them denoting their spawn time, multiple little swords or explosion-icons denoting the multi-hit ability, and... whatever determines when they do or don't attack. Plus, something like a line or a flashing bit of around the dream bar denoting how much spawining an enemy will deplete it.

This is a lot of suggestions, but I do think this is the best concept of the games I've played so far. And the art and sound all look wonderfully fitting. It's just that I can't figure out how I should play if I want to win that holds it back from being the best game I've seen thus far.

very cool two player concept. I like the art and the gameplay a lot! 
The atmosphere of the game is very calm which makes it quite relaxing to play !

Great job!

Sweet idea. It's just a bit boring after some time. :)