Great concept! Really liked the idea of the fusion.
Some things to note. I needed a restart hotkey, the button on the screen isn't a good solution because it breaks my flow and I had to reach for the mouse. Also, the "are you sure you want to restart" was just annoying. Of course I want to if I grabbed my mouse for it.
The enemies have a bit too much hp when fighting them not combined. If while combined you kill them easily then it doesn't matter if they have a lot of HP.
But, it was an enjoyable, creative game. good job!