1)Check the devices are on different enclosures.

# vxdmpadm listenclosure all
ENCLR_NAME        ENCLR_TYPE     ENCLR_SNO      STATUS       ARRAY_TYPE     LUN
================================================================================
disk              Disk           DISKS                CONNECTED    Disk        2
pp_emc0           PP_EMC         000290301414         CONNECTED    A/A         4
ams_wms0          AMS_WMS        75050205             CONNECTED    A/A-A       8
emc1              EMC            000290301414         CONNECTED    A/A         4

# vxdmpadm listctlr all
CTLR-NAME       ENCLR-TYPE      STATE      ENCLR-NAME
=====================================================
c0              Disk            ENABLED      disk
c4              EMC             ENABLED      emc1
c4              AMS_WMS         ENABLED      ams_wms0
emcp            PP_EMC          ENABLED      pp_emc0
 

# vxdisk -e list |grep jmdg
emcpower0s2  auto:cdsdisk   d1           jmdg        online               emcpower0c       -
emcpower1s2  auto:cdsdisk   d2           jmdg        online               emcpower1c       -
emc1_00e6_1  auto:cdsdisk   d4           jmdg        online               c4t3d48s2        -
emc1_00e7_1  auto:cdsdisk   d5           jmdg        online               c4t3d49s2        -

#2) Create a volume

# vxassist -g jmdg make testvol 1981m alloc=d4
# vxprint -qthg jmdg
dg jmdg         default      default  28000    1295910123.54.nodeA
dm d1           emcpower0s2  auto     2048     4121856  -
dm d2           emcpower1s2  auto     65536    4058368  -
dm d4           emc1_00e6_1  auto     65536    4058368  -
dm d5           emc1_00e7_1  auto     65536    4058368  -
v  testvol      -            ENABLED  ACTIVE   4057088  SELECT    -        fsgen
pl testvol-01   testvol      ENABLED  ACTIVE   4057088  CONCAT    -        RW
sd d4-01        testvol-01   d4       0        4057088  0         emc1_00e6_1 EA

3) Mirror it across an enclosure

 # vxassist -g jmdg mirror testvol mirror=enclr

 # vxprint -qthg jmdg
dg jmdg         default      default  28000    1295910123.54.nodeA
dm d1           emcpower0s2  auto     2048     4121856  -
dm d2           emcpower1s2  auto     65536    4058368  -
dm d4           emc1_00e6_1  auto     65536    4058368  -
dm d5           emc1_00e7_1  auto     65536    4058368  -
v  testvol      -            ENABLED  ACTIVE   4057088  SELECT    -        fsgen
pl testvol-01   testvol      ENABLED  ACTIVE   4057088  CONCAT    -        RW
sd d4-01        testvol-01   d4       0        4057088  0         emc1_00e6_1 EA
pl testvol-02   testvol      ENABLED  ACTIVE   4057088  CONCAT    -        RW
sd d1-01        testvol-02   d1       0        4057088  0         emcpower0 ENA

4) Create the filesystem and mount it

# mkfs -F vxfs /dev/vx/rdsk/jmdg/testvol
    version 7 layout
    4057088 sectors, 2028544 blocks of size 1024, log size 16384 blocks
    largefiles supported

 # mount -F vxfs /dev/vx/dsk/jmdg/testvol /testvol
 # df -k /testvol
Filesystem            kbytes    used   avail capacity  Mounted on
/dev/vx/dsk/jmdg/testvol
                     2028544   17990 1884902     1%    /testvol

 5) Check the free space for the new devices in this diskgroup
# vxdg free
GROUP        DISK         DEVICE       TAG          OFFSET    LENGTH    FLAGS
jmdg         d2           emcpower1s2  emcpower1    0         4058368   -
jmdg         d5           emc1_00e7_1  emc1_00e7_1  0         4058368   -

6) Increase this volume and use the new devices but ensure it is mirror across enclosures.   This volume will be over 1g in size and  the layout will be change to a layered volume.  Therefore it will be converted to a stripe-mirror or concat-mirror volume depending on the current layout.

This is documented in the man pages for vxassist.

Extract from vxasssist man pages:-

The attributes stripe-mirror-col-trigger-pt and stripe-mirror-col-split-trigger-pt control the selection. They can be set in /etc/default/vxassist. Volumes that are smaller than stripe-mirror-col-trigger-pt are created as mirror-stripe, and volumes that are larger are created as stripe-mirror. If vxassist creates the stripe-mirror and the columns are larger than stripe-mirror-col-split-trigger-pt, the individual subdisks are mirrored instead of mirroring the columns of the stripe. By default, both of these attributes are set to one gigabyte.
The disk group must be created on a 3.0 or later release to use the new layouts, but older disk groups can be updated. See the vxdg upgrade command for more information on upgrading disk groups.

Therefore we need to create a called  vxassist file in /etc/default 

 # cat /etc/default/vxassist
stripe-mirror-col-trigger-pt=2000g   <== make this value very large
stripe-mirror-col-split-trigger-pt=2000g

We have 2 new disk added to the diskgroup
 # vxdg free
GROUP        DISK         DEVICE       TAG          OFFSET    LENGTH    FLAGS
jmdg         d2           emcpower1s2  emcpower1    0         4058368   -
jmdg         d5           emc1_00e7_1  emc1_00e7_1  0         4058368   -

7) Increase the size of the volume across enclouures
 # vxresize -g jmdg testvol 3900m mirror=enclr

# vxprint -qthg jmdg
dg jmdg         default      default  28000    1295910123.54.rdge220r-01
dm d1           emcpower0s2  auto     2048     4121856  -
dm d2           emcpower1s2  auto     65536    4058368  -
dm d4           emc1_00e6_1  auto     65536    4058368  -
dm d5           emc1_00e7_1  auto     65536    4058368  -
v  testvol      -            ENABLED  ACTIVE   7987200  SELECT    -        fsgen
pl testvol-01   testvol      ENABLED  ACTIVE   7987200  CONCAT    -        RW
sd d4-01        testvol-01   d4       0        4057088  0         emc1_00e6_1 EA
sd d5-01        testvol-01   d5       0        3930112  4057088   emc1_00e7_1 EA
pl testvol-02   testvol      ENABLED  ACTIVE   7987200  CONCAT    -        RW
sd d1-01        testvol-02   d1       0        4057088  0         emcpower0 ENA
sd d2-01        testvol-02   d2       0        3930112  4057088   emcpower1 ENA
v  vol1         -            ENABLED  ACTIVE   1280     SELECT    -        fsgen
pl vol1-01      vol1         ENABLED  ACTIVE   1280     CONCAT    -        RW
sd d4-02        vol1-01      d4       4057088  1280     0         emc1_00e6_1 EA
v  vol2         -            ENABLED  ACTIVE   64768    SELECT    -        fsgen
pl vol2-01      vol2         ENABLED  ACTIVE   64768    CONCAT    -        RW
sd d1-02        vol2-01      d1       4057088  64768    0         emcpower0 ENA