We use this hardware for our remote offices and this is the configuration that we use to maximize space.
Node 1 - One aggregate of 9 disks (RAID-DP) with one spare. This will give you approximately 5TB of usable space on the aggregate (7 data disks, 2 parity disks). If space it really a concern you _could_ go RAID-4 here but I wouldn't recommend it for production data.
Node 2 - One aggregate of 2 disks (RAID-4) with no spares. You will get alerts about there being no spares available but you can stop those by setting 'options raid.min.spare_count' to '0'