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It is possible. People typically use a shared version control repository (say, github, for example, but anywhere you can both access the same git repository would work). You pull changes and you push changes to that git repository, and that way you both get each other's changes. And with this comes to the drawback of GMS current project structure which is not well suited to handling merges of independent changes. You will get more conflicts than you might like when both of you manage resources (rooms, objects, sprites, etc.) independently. The least problems are probably had when only one person creates the base resources, and then both just change code or edit the created sprites, etc.