You won't be able to have one single aggregate assigned to two controllers.
One aggregate is assigned to only one controller at a time. It can be re-assigned, but not used by two controllers.
With your system, you should be able to use ADP, partitioning, which at least, should allow to use the 12 disks on the 2 systems by creating partitions. One small partion of the disk assigned to the root aggregate, one partition to controller 1 and the last partition to controller 2.
If you want to have the maximum capacity on only 1 aggregate, I would recommed to go to an Active/Passive configuration where the second controller doesn't have any data aggregate (only root aggregate), and would be only active in a Failover situation.