Hi Ohmk,
Only one person can work on a specific map at a time, but you can create multiple separate sublevels and work independently on them, with both levels loaded into a third map that neither of you would typically have checked out.
Go to Window…Levels, which will open the Levels panel. From there you can right-click and choose Add Level to create new empty levels for you to work in. You’ll probably want to set them to Always Loaded.
Cheers,
Michael Noland