Hi, I'm currently planning to develop 2D game with my brother as hobby.

Me: computer science major, but no game developing related experience. I can do C, C++, Python, web related techs(html, css, javascript, php, mysql, etc). Very brief knowledge about network. Algorithm & data structure knowledge. Familiar with mathematics, physics.

Brother: Good at drawing, but no game graphic related experience. Only he can do is just drawing on paper/tablet. Want to learn game graphic skills(animation, dot graphic, sfx, etc). Python beginner

To develop game with two-man system(programmer/graphics artist), what skills is additionally needed, respectively? We're looking for your advice. If there are useful article or contents, also thankful! ?

You need to learn how to learn libraries and engines, unless your goal is to learn how to implement low level functionality. Learn to use game libraries and graphics libraries and audio libraries and the rest.

Your brother needs to learn all about game art, in whatever style matches the game you're building. 2D animated pixel art is quite different from 3D animations (which can be used quite easily in 2D games).

The math of 2D world manipulation is algebra and trig, often taught before high school graduation. The math of 3D world manipulation is linear algebra, usually taught in college and sometimes a CS requirement. Hopefully you have both of these covered in your education. People can be self-taught in what they're missing, but it is certainly easier to have guidance.

For hobby projects, just start into it and figure out what you don't know. Start with an existing engine unless your goal is to write everything from scratch. Try to do something, and if you can find a way to do it you're good. If you can't, learn whatever you need to make it work. It won't be the best, but it will be something that you understand.

Set realistic expectations, as this won't be a 40/week multi-year project filled with industry veterans. Think about the complexity of games from the 1980s, and while the images can be more modern, the logic they used is within reach of beginner hobby projects.