This is a really unique and fun game with cute graphics and sound, but there were a few things that made it kind of frustrating for me in single-player mode. I got stuck on level 4 because of them.
My first issue is that having to move both characters at once is kind of a problem when there are hazards to avoid. In level 4, if I missed a throw, inevitably one character would end up in the water multiple times while I had the other run after it. If I had to step off the switch prematurely because I was moving the other character, I had to spend several throws re-orienting myself onto the switch. Would it be possible to hold Ctrl / Shift to move only one at a time? Since the game is meant for two players, I'm assuming that moving at the same time isn't meant to be a core part of the challenge.
My other issue was just that I had trouble seeing exactly where the frisbee would go on a long throw. Level 4 in particular demands precision over long distances, since missing a throw can cause the frisbee to become irretrievable, or (as I mentioned) cost you several strokes as you fumble after it. I think a tracer would help a lot - it doesn't have to go all the way out, just a short distance past the cursor would help visualize its path. Do you think this would make it too easy? On my last attempt at level 4, I failed because the frisbee landed a few pixels away from the goal and I couldn't retrieve it.
Aside from those issues though, I had fun playing, and I wish I could give the coop mode a try since it seems like it would be even more fun. Good work!