The concept feels like it has something there, but it never really... expands upon it.  From the bit I did.  It's lacking a lot of feedback, and it's very slow paced.  Especially considering you have a grenade launcher.  Movement was very floaty and once again without any feedback.  Some finer points:

• Like I said the idea seems interesting but the game itself seems completely counterintuitive to it.  You're given a grenade launcher, and then told to only fight one enemy at a time.  Which takes away the main fun of a grenade launcher.  I feel like changing up the weapon if you're being made to fight a single enemy at a time would help.  A pistol or something similar.
• Although my recommendation would be to tweak the main mechanic and rather than making them invincible when they're together just increase their health and damage.  Maybe have them combine into different types of enemies etc.  That way it could vary up the gameplay while still fitting the theme and the main mechanic.  Then when you use the balloon gun, it could "separate" them which would weaken them and make them easier to kill.  
• As I said as well movement felt very floaty which didn't help too much.  I feel like increasing gravity or something to that effect would help there, though not sure. 
• The game felt very slow paced overall, though that's down to a ton of factors so I can't really pinpoint one main one to comment on here.

That's about it I think.