Use multiple mbox locations

[bapt x]

Hello,

With GNU mailutils on Debian 9, we receive mails in /var/mail/<username>
but when we read mails with the "mail" command, they are moved in a file
~/mbox: "Saved 1 message in /home/<username>/mbox".
I use Roundcube webmail with Dovecot but if I also use use the "mail"
command to read emails, Roundcube will not display old messages anymore
since they were moved to ~/mbox.
So is their a way to tell Dovecot that we want to lookup for emails both in
/var/mail/<username> and ~/mbox?
I saw https://wiki.dovecot.org/Namespaces#Mixed_mbox_and_Maildir but I it
did not work if when I tried to add 2 times a mbox location.

Thanks for your help.