iMac's SSD slot
For the first time, Apple sells a 27-inch iMac with both the 256GB SSD and one of the hard drive options pre-installed.
(It is not possible to order an iMac with two SSDs or two hard drives.)
OWC took apart a new 27-inch iMac to see where the SSD option is installed. The red box indicates the location of the SSD bracket. Photo courtesy of OWC.
It seems that the bracket to hold the SSD in place isn’t installed unless an SSD is installed by Apple.
Then again, just because something's difficult doesn't mean it's impossible. From the OWC blog:
Of course, you can still install that second drive on a non-SSD iMac if you:
1. Connect a data cable to the empty SATA port on the back of the logic board (a process requiring the removal of several screws and partial removal of the logic board.
2. Get a Y-cable that goes from 1 SATA male power connector to 2 SATA female power connectors. (This step is actually pretty easy.)
3. Find a way to secure your 2.5" drive in the space that the carrier would normally hold it.