Firstly, congrats on your first game jam! Just having something published is a great accomplishment in itself :).
I like where your idea is going and would encourage you to spend more time experimenting on it and building more levels and such. Your visual/audio effects on the abilities felt nice and powerful too.
Real quick: one bug (not quite a bug but oh well) I encountered was if you walk left you fall off the map forever :(.
My biggest suggestion is to give some visual indication to your players of the systems in your game.
Health for example, I didn't know if I could get hit twice overall or if it was a different health bar for both characters. Having a little health bar, or maybe just showing some visual cracking on the ice character or the fire character is dimmer can help clue the player in on what their situation is like.
Another example would be which character you're controlling. Sometimes I found when I had them both near each other I got confused which one I was and ended up using the wrong power. You could have something simple like a halo behind the one currently in control.
Lastly, giving the player an understanding of the time for fusion and the cooldown. All of these things help the player make snap decisions and strategize while playing your game :).
Another suggestion is related to how quickly the enemies attack you. It was frustrating a bit because I couldn't really tell how close I had to be before they'd start attacking me, so maybe some indicator of this. My first thought was like in Metal Gear, the enemies are alerted with that exclamation. You could have that with a quick delay before it starts firing to give the player something to react to.
(As a little hint from a fellow dev. I often forgot to switch my menus back while I was working on them, so I wrote a script that stores 1) a list of all the menus and 2) the specific menu I want to start. Then when that object starts, it disables all menus and enables the specific start menu. That way, no matter which menu I was working on and forget to disable, when I play it'll always bring me back to the right one. Saved me a ton of hardache in the long run!)
Cheers!