Most arrays and RAID controllers allow to split the raw disk space into several logical volumes which are then presented to the operating systems. Assuming your controller supports this, I'd create a large RAID5 set with all 6 disks and split the resulting RAID set in 2 logical volumes, where each of them is smaller than the maximum supported LUN size (2TB - 512Bytes). This way you get the most space out of the disks in a single RAID5 set.
With this amount of disk space you may also consider to use one of the disks as a hot spare!
André