Very hilarious entry towards the ending when the boss starts spamming bullets! Let's go through the details:

The Good:

There is a surprising amount of visual polish, some of which I am still figuring out how to implement. The way you implemented the trailing effect when you move your character is a nice small touch that lets you follow the player movement. Swapping character has a brief particle effect but I think it'd be beneficial to have it more apparent.

The very subtle, but apparent bloom is a nice post-processing touch, something I'm looking to implement in my future games too and it's done quite well here, giving the game a simple charm!

To improve:

I think you have the foundations for a core game loop, a survival game with potential crazy over-the-top bullet patterns like those seen in bullet hell games (touhou comes to mind).

If you're designing your game as a survival game in mind, you can consider expanding on that concept by adding a timer (for how long you've survived), or if the waves are limited, then add a wave number to show how close the player is to winning.

You also took a unique approach to including two players which I think is not fully realised in this game, it's an interesting design choice which could be expanded a bit more to make for a cool game. What if the boss has orange and blue bullets, where blue bullets will hurt only hurt the orange character and the orange bullet will only hurt the blue bullet?

You've also designed your game to have no live system, ie. any character that gets hit dies immediately. I think if you lean into this mechanic as well, and considering all the suggestions above, you can create a rage-game kind of experience, where players have to mind juggle multiple characters, to evade multiple, dynamic bullet hell patterns. Then insert some good EDM/Speedcore music and well, that's an ULTRA Game!

Overall, very pleasant surprise. Good one!