Top Notch!
First, I am super impressed by the completeness of this submission. A tutorial. A bunch of different enemy types that all work with the joining mechanic. Lighting effects. Crisp, non buggy controls. Screen shake. SFX for pretty much everything. An Epic sounding and highly varied musical composition. This is just fabulously juiced up for 48 hours of work!
Next, I think this is an excellent take on the theme, both the joined and unjoined states have meaningful gameplay consequences, and you get to play around with it a lot by controlling all the different enemy types.
The only criticism that comes to mind is that when the level first loaded up for me, it took a while for an enemy to spawn in, so I was confused about what to do for a few moments. It cleared up fast though.
Great work!!!