[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)

Steps to Reproduce:
1.mkdir /tmp/{1,2,3}
2.mkdir /tmp/1/test1 /tmp/2/test2
3.mount -tunionfs -o dirs=/tmp/1/test1=ro:/tmp/2/test2=ro unionfs /tmp/3
Actual Results:  
no mount happens

Expected Results:  
test1 and test2 dirs in /tmp/3