Splitting up the tasks so you're not stepping on each others toes is the smart way to go.
TIP: Whoever works on the database keeps the "Master" copy.
WHY: When you save your game, all of the database files get saved, whether changed or not. Maps only get saved when they change.
If your friend works on maps 12-16, he can just send you the MAP012 - MAP016.rxdata files & you can update your copy.
If he adds new maps, you also need the MapInfos.rxdata file.
You can send him the database files you change (One file per database tab)
The only time you run into trouble is when he wants to update a map that you've edited (added events, etc.). Then you have to do a manual comparison & merge the changes from one RMXP session to another. (HINT: If you didn't realize, you can open more than one session of RMXP at a time.)
Another option is to use a "Version Control" system. This just kinda automates keeping track of who's working on what, and what has changed. But you need a common repository that you both can access to check-in & check-out files from. And you still have to do manual comparisons when there is a discrepancy.
Be Well