It's not yet possible to split revenue from one game among several creator accounts on Itch, sorry. People ask for it all the time, so we hope to add it soon.

Your other question isn't about Itch, but anyway. It depends. With GitHub it's best to try it for a small project and see if you like it. If you have many large assets, it might not be a good idea. And it can be complicated to use. Also, Microsoft bought them last year. But you have many tools to discuss with your team mates, write documentation, make plans, combine your code and so on. And you don't need to use Git, many things work from the web interface.

Edit: best to ask about GitHub in General Development.