Combining the units to form one big array or unit is not possible on the ReadyNAS units. There were Expansion Chassis available before like the EDA500 but everyone has different experiences on using Expansion Chassis.
IMO, it is always better to have different NAS and different volumes on your arsenal so as to better implement availability on your Network. But if you want that feature you can always get to write on the Ideas exchange board, infact there was a request before but I'm not sure if it has gotten enough attention yet.
