Hey I love the wall climbing physics and double jump. Splitting the characters between one who is good at platforming and one for combat is an awesome idea. The dynamic music to go along with each character is also well-done, no complaints. This is an interesting choice for duality, to be certain. Reminds me of the character swapping features in platformers like Mega Man X8.
There are bugs with the wall climbing. More I-frames and a hurt/death animation are definitely needed too. And the fireball, yes, should be faster. But there's a foundation to go off of, for sure.
The visual assets are where things get iffy. In a vacuum, the player sprites are well-done. The background is fine. The spikes are distinct. However, they clash a lot. The backgrounds look almost like an Atari game's but with parallax scrolling, while the player sprite looks like it's from a 16-bit platformer. The spikes are perhaps a bit too clean and not very pixelated.
Hitting a consistent style is hard but I believe in you to reach that point.