I really liked this game. The music fits very well and the gamplay is fun. I have a couple of suggestions:
1.Try to add some particle system when you collect the stars
2. You can add camera movement so you could see both players at the same time, Brackeys have a video about it here https://www.youtube.com/watch?v=aLpixrPvlB8&t=90
3. You can add a variables to your ToString method on the clock, like if you write .ToString("00") it will return '09', '08', '07'...
4. Make the colliders of the stars bigger so it'll be easier to collect them.
Great job! :)