[root@ivanovpc tmp]# mount -tunionfs -o dirs=/tmp/1/test1=ro:/tmp/2/test2=ro unionfs /tmp/3
mount: wrong fs type, bad option, bad superblock on unionfs,
       missing codepage or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

[root@ivanovpc tmp]# dmesg | tail
unionfs_read_super: error while parsing options (err = -22)