Super underrated, I love the main twin mechanic, you could do so much with it.
I heard you wanted to make it into a bigger game, so here's my suggestions.
- You should make all the different skull powers more clear what buttons do what.
- Not required and entirely my opinion, but it would be cool if you unlocked more skull powers when you play. You could make them so you can only have a couple equipped at once, and then there would be something to work towards on new playthroughs.
- I think a small screenshake and more particles could polish this a lot. Also more sounds and everything.
Overall really great job! I look forward to seeing it updated if you do