1) How are you doing it now?
Dividing the work depends on what each others strengths are. One person could do all the mapping & event creation, while the other works on music, graphics, story, etc...
If you both want to work on the mapping, database, events then you'll need to do some "Version Control". If it's just 2 of you, you can do this manually. All you need is a common place that you can both access to store the 'master' data. Then you each download any new data each time you go to work on the game, then upload any data you changed when you're done for that day/session.
You'll want to plan, so you're not working on the same thing at the same time. Make a list of 'tasks', and when you start a task, put your name on it so the other guy knows you're working on it, then he can just grab the next task in the list & put his name on it.
If you are both mapping & creating events, you'll want to assign maps as well as variables & switches.
variables & switches are organized in RMXP in groups of 25, so do the same thing. You get 1-25, he gets 26-50, and when you need more create another list to keep track, and assign blocks as needed.
Do the same thing with maps. Take the game as it is last saved before he leaves, and add a whole bunch of blank maps. then, assign groups of maps to each person. If you have your world all planned out ahead of time, you can pre-label (Name) each empty map to make it easier.
If he works on map 26, then uploads it or sends it to you, you can just copy it into the /Data folder overwriting the existing (blank) Map026.rxdata file, then open your game & Presto! there it is.
Each tab on the database has it's own .rxdata file, so as long as you don't both change the same tab at the same time, you can transfer those files as well.
Again, planning ahead will save a lot of headaches. You can design your world, database, story, script (sequence of events), characters, graphics, before you start building anything in RMXP.
You could also use a "Version Control System" like SVN (SubVersion), CVS, SourceGear, et.al.
This type of system allows you to create a "Repository" which serves as the master copy, and then each person creates a copy of the Repository. When you work on a specific file, you "Check it Out", which puts a lock on it so the other people will see a warning if they try to work on it while you have it checked out.
Probably overkill if it's just 2 of you.
Be Well
well! those were very helpful tips!