I guess like this:
First of all you need a friend you'd make the game with.
Then both of you need to own a copy of that particular rpg maker, especially if both of you will be working in it.
The rest is communication, making maps, sharing them with each other and implementing them to the main project. Just like you do any other project with friends. One does one part, the other does the other part, then you merge these two parts together and connect them to the main project. Don't tell me you have never done any collaborative work.