Symbolic links will only work with shell users.
We've thought about various ways to unchroot certain FTP users without causing problems. The main thing is that FTP users get very upset and confused when they realize they can get to "/" and see everything there - they think that it's some sort of security hole and write support. There are ways to exclude individual groups, but we haven't really messed around with this much.
Sooooooo.... the best options are:
1) Have one user only edit group-writable files from the shell (or using scp / sftp), and have the other user be the main FTP user. You can symlink /home/user1/example.com to /home/user2/example.com.
2) Create a third "ftp" login, and have both users use this login to update files.
3) Create one user with their home dir nested inside the other user's home dir (/home/user1/user2). Then symlink /home/user1/user2/example.com/ to /home/user1/example.com.
These are the three ways I know of that will work. I personally prefer "1" and "2".