[CentOS] Software RAID10 - which two disks can fail?

Hi All.

I have a server which uses RAID10 made of 4 partitions for / and boots from
it. It looks like so:

mdadm -D /dev/md1
/dev/md1:
        Version : 00.90
  Creation Time : Mon Apr 27 09:25:05 2009
     Raid Level : raid10
     Array Size : 973827968 (928.71 GiB 997.20 GB)
  Used Dev Size : 486913984 (464.36 GiB 498.60 GB)
   Raid Devices : 4
  Total Devices : 4
Preferred Minor : 1
    Persistence : Superblock is persistent

    Update Time : Mon Apr  7 21:26:29 2014
          State : clean
 Active Devices : 4
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 0

         Layout : near=2, far=1
     Chunk Size : 64K

           UUID : 1403e5aa:3152b3f8:086582aa:c95c4fc7
         Events : 0.38695092

    Number   Major   Minor   RaidDevice State
       0       8        6        0      active sync   /dev/sda6
       1       8       22        1      active sync   /dev/sdb6
       2       8       54        2      active sync   /dev/sdd6
       3       8       38        3      active sync   /dev/sdc6

As far as I know raid10 is ~ "a raid0 built on top of two raid1" (
http://en.wikipedia.org/wiki/Nested_RAID_levels#RAID_1.2B0 - raid10). So I
think that by default in my case:

/dev/sda6 and /dev/sdb6 form the first "raid1"
/dev/sdd6 and /dev/sdc6 form the second "raid1"

So is it so that if I fail/remove for example:
- /dev/sdb6 and /dev/sdc6 (different "raid1's") - the raid10 will be
usable/data will be ok?
- /dev/sda6 and /dev/sdb6 (the same "raid1") - the raid10 will be not
usable/data will be lost?

I read in context of raid10 about replicas of data (2 by default) and the
data layout (near/far/offset). I see in the output of mdadm -D the line
"Layout : near=2, far=1" and am not sure which layout is exactly used and
how it influences data layout/distribution in my case :|

I would really appreciate a definite answer which partitions I can remove
and which I cannot remove at the same time because I need to perform some
disk maintenance tasks on this raid10 array. Thanks for all help!

BR,
Rafal.