If I understand correctly, you want to make one entry with your teammate, and he wants to add a separate entry on his own? If so, I see nothing wrong with it:) As long as you basically don’t submit the same project. If the projects share significant portion of the code, to avoid any accusations just be transparent and describe what’s common and what’s different between your team game and your friend’s solo game in the entry description. Thank you for asking!
If anyone has some strong opinion about this, also feel encouraged to leave a comment:)