You give user1 and user2 one directory to share on the remote server ? with group write permission.
chroot is used to set up a restrictive environment (a mini root file system) then within there a /home/shared_directory could sit.
The first error is because you set the chrootdirectory to the users home directory (everything in chroot should be owned by root and not writable).
The second error you are setting the chroot to /sftp/username
Here's a similar question.
http://unix.stackexchange.com/questions/108778/chroot-sftp-users-who-require-access-to-multiple-directories-under-same-parent-f?rq=1Chroot SFTP users who require access to multiple directories under same parent folder