Assuming a commonly used linux distribution and sshd service running.
1) If you want two users to be able to create, delete and modify files under the same directory, then just create one account and provide the credentials to both people. That's the easiest way, however...
...if you want to log their activity separately, e.g. who logged when, then create two users with the same group and assign the same home directory to both of them. E.g.
chown nobody:sftp /home/sftp
chmod 770 /home/sftp
useradd -d /home/sftp -g sftp sam
useradd -d /home/sftp -g sftp tom
if users sam and tom already exist and have their own primary group, then
chmod g+s /home/sftp will force files created under this directory to inherit its group id.
Check this other post for how to force a default
umask under sftp, so files uploaded are group read/write-able.
2) If you want only one user to create, delete and modify the directory contents and a second user to have read-only access. This doesn't really need any special handling, normal users can read contents from other users' directories. The second user will just have to specify or cd to the target directory (however, you may assign the same home directory to the second user).