I really like the concept and the controls feel smooth and allow for a good amount of precision which is always important for a platformer.
Some minor issues I had:
- Some of the levels felt a little slow-paced, especially sections that required frequent switching between the characters
- The collision on walls and the side of platforms was sometimes a little wonky, causing the characters to get stuck in midair to the side of a platform. I managed to abuse this a couple of times to glitch onto platforms I couldn't reach with the jump alone.
- In my opinion it would be better to have one switch button instead of two. I had some issues mixing them up, especially in platforming sections where you want to make a quick, well-timed switch.
- From a purely ergonomic perspective I/U + Arrow Keys is not a good choice of buttons. On most keyboards, they are too far apart to control with one hand but too close to comfortably control with two hands. It doesn't matter too much in a short game like this, but in a longer one, keeping the hands in position to press both at a time is likely to put strain on the player's hands and forearms. If you ever plan to make a longer game, that's something to keep in mind.
These are all just minor issues, though, and most are easily fixable. Good job!