This is possible, but honestly not worth the effort. Worst case scenario is the system board fails and you're down for up to 4 hours until a replacement is sent out (unless you only have Next Business Day). It could take longer than that just configuring and getting clients to connect to the inactive controller.
When the active system fails you will have to move all network connections to the inactive controller and reassign ownership of all the disks to it. All of your clients will have to reconnect to the new controller because it will have no session information at all (the reason most customers go for at least the active/passive setup). If you have FCP/iSCSI clients you have even more issues getting them to connect to a system likely to have different WWNN/WWPNs. Once you have this working, you could set aside the replaced controller to handle another failure. The problem there is your AutoSupport history is tied to the system serial number. Every time you have a system failure, your history starts over again which could get confusing for NetApp Tech Support.
Unfortunately, there is no middle ground. Either a system is completely standalone and you accept downtime for failures and maintenance or it's an active-active configuration that consumes drives, but significantly reduces downtime.