Great idea on this one - i like the mechanics. I found this game to be really hard & couldnt finish it. Switching between them & having each heart its special ability (shoot OR shield) was awesome. I might suggest making enemies melee to be less hardcore & instead of shield have some area effect or area shield for a brief moment.
The game is unfairly hard, and it doesn't end. I forgot to add some kind of score mechanic. I didn't have much time to work on making the enemies less hard or making the shield mechanic more practical. It still has a lot of bugs, and apparently the shield doesn't work for some people, so I'm glad it worked and that you liked it. Thanks for playing! :D