Theoretically, it seems to be feasible. However, 2012 iMacs do have SATA blade port (not the PCI-e blade SSD after 2013 models) and SATA port, those can be paired as RAID 1.
My question would be - what would be the gain if Mac Time Machine (even multiple time machines) can be used for solid backup. 2T blade SATA SSD can be a rarity with possible sticker shock.
Another option will be - use an adapter to transform the blade SATA into regular SATA port via this. Then you will have the ability to put two 2T SSD from the newest 2.5" SSD.
You have to use disk utility before OS installation, such as this link did illustrate the steps.
Good luck
Sam