I am planning to use two expanders in one signel backplane, such as 846E2,847E2, 216E2 backplane. It's saying the support on dual expanders to have failover mode. I cannot find any controller in supporting this redudant failover.
You can simply consider to use Adaptec RAID 31605 which does support Redundant path failover. For failover mode, it relies the support on controller and itself firmware capability.