You can only do this if you work in tandem.
– You start a project and share the url with him
– He loads that shared game, adapts it, saves it as his own project and shares that with you
– You can then load his game, adapt it, and save it over your original game
After that you both have the shared url of the project of the other person, and can load it and save it as your own.
But remark that you can’t work on the game simultaneously. So you will have to let the other person know when you start and stop working on it.