Use dual-split backplane in both enclosures for the three AIX partitions and the IBMi partition.
Put each partition on a dedicated controller using the triple-split backplane option in both enclosures.
Use the triple-split backplane option in one enclosure for the AIX partitions, and the default dual-split backplane for the IBMi partition in the second enclosure.
Use the triple-split backplane option in one enclosure for the AIX partitions, and the non-split backplane mode in the second enclosure for the IBMi hosting partition.