Great work team! This is absolutely wonderful. The visual design is appealing and extremely polished--the parallax in the background and the endearing game-over crying animation are two of my favorite details. The concept is extremely novel as well, and controlling both characters with the up and down arrow keys is surprisingly intuitive. The audio ties it all together though--the way the animations line up with the music's beat is sensational, and the melody coming in right after the tutorial is a great touch.
As with below commenters, my principal hang-up is that I'm getting absolutely smoked. I couldn't hit double digits no matter how hard I tried. I think that keeping the balloon below the danger zone while jumping is challenging enough to start out--maybe bring in the birds later, or have an early section with just birds! Overall, though, this is a stunningly complete package. Amazing work!