Is it possible to use Matrix RAID for having two RAID5 volumes on the same 6-disk array? (assuming ICH9R or higher)
Yes its possible.
Keep in mind a 6 disk RAID with two RAID 5 arrays on it will fail if two disks fail out of the 6 disks for both arrays. You can opt to have two RAID 5 arrays each with 3 disks to allow a disk to fail in each.