#/bin/bash

marcfs=/path/to/marcfs/binary
dest_dir=/path/to/dest/dir

# Make all needed dirs
mkdir -p $dest_dir /tmp/markfs/cloud1 /tmp/markfs/cloud2 # /tmp/markfs/cloud3...

# Init marcfs
$markfs /mnt/MailRuCloud1 -o username=First@mail.ru,password=pass_for_this_account1,cachedir=/tmp/marcfs/cloud1
$markfs /mnt/MailRuCloud2 -o username=Second@mail.ru,password=pass_for_this_account2,cachedir=/tmp/marcfs/cloud2
# ...

# Mount markfs dirs in
mhddfs /mnt/MailRuCloud1,/mnt/MailRuCloud2 $dest_dir

# Now we have union of Sirst's and Second's clouds at $dest_dir!