The whole equals the sum of its parts, that's what they teach you in math. I stand by this notion in math, but in everything else Aristotle said: "The whole is greater than the sum of its parts".
Have you ever wanted a job where they sent you to a place where you sat and played with other people while a psychologist wrote stuff? No? Well, they do that. They want to see how much of a team player you are. Why? Because 1 person + 1 person can do the work of more than 2 people when both play ball and coordinate correctly.
Co-designing a game is just like that. You need to know the dynamics of the team and set things so that both sides shine. Know your stronger sides and weaker sides. Is one of you better in pitching? Is one of you better at graphic design? use that and also coordinate the work according to those strengths. Make sure every side has a task in a "to do" form that will move the project onward.
Another important thing when working in a team is never losing sight of the end goal: getting a game to the table or to production. It is great if one of the designers assumes the role of "manager" by always having things organized, knowing what should be the next step. This should be the more experienced designer and / or someone who led projects. Find it hard to do? Use a project managing software (like MS project for 7$ a month). End every meeting with arranging the tasks and timelines and replace the need one manager.
Also, use a cloud service. I prefer Google Drive because it updates automatically and also has every tool needed (documents and sheets basically). That way you can work in parallel and quickly while even seeing each other's work being done. All this without clashing in file versions and problems with different softwares. It is 100 times better than sending endless emails with gamerules_v43.docx, or was it v44?
But more than everything else, know yourself. Do you tend to work well in a team? Because if you do, go ahead and try it out! If you don't, just don't go into it. I know people who burnt bridges by trying to cooperate with others while being completely unable to receive criticism even when it was constructive. So don't unmake friends, we are in the small world of board games, after all is said and done.
Find your path, work to your strengths and never lose sight.
Now go design.
Pini!
In the picture a sort of coordination I will never achieve. For the lack of trying and skill :)