How do I share folders ?

[chaouche yacine]

Hello dovecot, 

First poster here.
What I want : give a.chaouche at algerian-radio.dz list and read permissions on a.chaouche at backup.algerian-radio.dz. Let's just discuss sharing the inbox then I can do the same for subfolders, in any. a.chaouche at algerian-radio.dz and a.chaouche at backup.algerian-radio.dz are two different users. Details about the domains aren't relevant here I guess.

My dovecot configuration, what I believe to be the important parts :userdb {
  args = uid=vmail gid=vmail home=/var/vmail/%d/%n/
  driver = static
}
namespace partage {
  location = maildir:%%h:INDEX=~/%%u
  prefix = shared.%%u.
  separator = .
  type = shared
}
mail_location = Maildir:~/
Both users can access their respective inboxes just fine, I only need to give a.chaouche at algerian-radio.dz access to a.chaouche at backup.algerian-radio.dz


Full output of doveconf -n :
# 2.2.13: /etc/dovecot/dovecot.conf
# OS: Linux 3.16.0-4-amd64 x86_64 Debian 8.3 
auth_mechanisms = plain login
debug_log_path = /var/log/dovecot.debug
disable_plaintext_auth = no
first_valid_uid = 100
log_path = /var/log/dovecot.log
mail_debug = yes
mail_location = Maildir:~/
mail_plugins = " quota acl"
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope encoded-character vacation subaddress comparator-i;ascii-numeric relational regex imap4flags copy include variables body enotify environment mailbox date ihave
namespace inbox {
  inbox = yes
  location = 
  mailbox Drafts {
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    special_use = \Trash
  }
  prefix = INBOX.
  separator = .
  type = private
}
namespace partage {
  hidden = no
  location = maildir:%%h:INDEX=~/%%u
  prefix = shared.%%u.
  separator = .
  type = shared
}
passdb {
  args = /etc/dovecot/dovecot-sql.conf.ext
  driver = sql
}
plugin {
  acl = vfile
  quota = maildir:User quota
  quota_warning = storage=95%% quota-warning 95 %u
  quota_warning2 = storage=80%% quota-warning 80 %u
  sieve = ~/.dovecot.sieve
  sieve_dir = ~/sieve
}
postmaster_address = root at algerian-radio.dz
protocols = " imap lmtp sieve"
service auth {
  unix_listener /var/spool/postfix/private/auth {
    mode = 0666
  }
}
service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    group = postfix
    mode = 0600
    user = postfix
  }
}
ssl = no
userdb {
  args = uid=vmail gid=vmail home=/var/vmail/%d/%n/
  driver = static
}
protocol lmtp {
  mail_plugins = " quota acl sieve quota"
}
protocol lda {
  mail_plugins = " quota acl sieve"
}
protocol imap {
  mail_plugins = " quota acl imap_quota imap_acl"
}

I don't know if it's possible to share folders with the doveadm acl command ?
root at messagerie[10.10.10.20] ~ # doveadm acl debug  -u a.chaouche at algerian-radio.dz shared/a.chaouche at backup.algerian-radio.dz
doveadm(a.chaouche at algerian-radio.dz): Error: Can't open mailbox shared/a.chaouche at backup.algerian-radio.dz: Mailbox doesn't exist: shared/a.chaouche at backup.algerian-radio.dz
 root at messagerie[10.10.10.20] ~ #