As the title.
Before buying a license, I just want to know the how to co-develop, or does C2 support co-developing?
Each person will need a licensed copy if you plan on using license features (families etc)
It doesn't support co-development.
But it doesn't need to as this is probably the better way to co-develop anyway : Tutorial: Collaborate using Github
It's the same as SVN I think.
But if my partner and I is changing the project at the same time, and I upload the project first, it's no way to merge our changes, right?
Develop games in your browser. Powerful, performant & highly capable.
Nope...
What has helped me in the past if you break project up into pieces and each work on his/her sections, then put together at end.