Hi
By default what has happens is the disks are split across the heads with each head owning half.
To change the owner of a disk you're going to need to get down and dirty at the command-line.
Connect via SSH/Telnet/serial/SP and type
disk show
You should get some that looks a bit this.
DISK OWNER POOL SERIAL NUMBER HOME
------------ ------------- ----- ------------- -------------
0a.00.14 myfiler01-c1(1234567414) Pool0 Z1N2L1JM myfiler01-c1(1234567414)
0b.00.15 myfiler02-c1(1234567663) Pool0 Z1N2L1ZF myfiler02-c1(1234567663)
0b.00.3 myfiler02-c1(1234567663) Pool0 Z1N2LA4X myfiler02-c1(1234567663)
0b.00.23 myfiler02-c1(1234567663) Pool0 Z1N2L23X myfiler02-c1(1234567663)
0a.00.4 myfiler01-c1(1234567414) Pool0 Z1N2LGYN myfiler01-c1(1234567414)
0b.00.19 myfiler02-c1(1234567663) Pool0 Z1N2L2FE myfiler02-c1(1234567663)
0b.00.13 myfiler02-c1(1234567663) Pool0 Z1N2L1XJ myfiler02-c1(1234567663)
0b.00.1 myfiler02-c1(1234567663) Pool0 Z1N2LH25 myfiler02-c1(1234567663)
0b.00.17 myfiler02-c1(1234567663) Pool0 Z1N2LBG8 myfiler02-c1(1234567663)
0b.00.21 myfiler02-c1(1234567663) Pool0 Z1N2L9DJ myfiler02-c1(1234567663)
0a.00.22 myfiler01-c1(1234567414) Pool0 Z1N2L3W2 myfiler01-c1(1234567414)
0a.00.0 myfiler01-c1(1234567414) Pool0 Z1N2L1Y9 myfiler01-c1(1234567414)
0a.00.12 myfiler01-c1(1234567414) Pool0 Z1N2LGRM myfiler01-c1(1234567414)
0a.00.10 myfiler01-c1(1234567414) Pool0 Z1N2L2TV myfiler01-c1(1234567414)
0a.00.18 myfiler01-c1(1234567414) Pool0 Z1N2LBJS myfiler01-c1(1234567414)
0a.00.20 myfiler01-c1(1234567414) Pool0 Z1N2L9H9 myfiler01-c1(1234567414)
0a.00.16 myfiler01-c1(1234567414) Pool0 Z1N2LHKW myfiler01-c1(1234567414)
0b.00.9 myfiler02-c1(1234567663) Pool0 Z1N2LGQM myfiler02-c1(1234567663)
0a.00.6 myfiler01-c1(1234567414) Pool0 Z1N2L91R myfiler01-c1(1234567414)
0a.00.2 myfiler01-c1(1234567414) Pool0 Z1N2L1DV myfiler01-c1(1234567414)
0b.00.5 myfiler02-c1(1234567663) Pool0 Z1N2LGE3 myfiler02-c1(1234567663)
0b.00.11 myfiler02-c1(1234567663) Pool0 Z1N2L8VH myfiler02-c1(1234567663)
0b.00.7 myfiler02-c1(1234567663) Pool0 Z1N2L92W myfiler02-c1(1234567663)
0a.00.8 myfiler01-c1(1234567414) Pool0 Z1N2L94C myfiler01-c1(1234567414)
You then need to identify the spare disks on one head you want to move to the other and type
disk assign diskid -s unowned -f
disk assign 0c.00.16 -s newfiler
Remember once a disk is allocated to an aggregate you won't be able to move it to another filer.
Shame you reseller / NetApp weren't more help at purchase.