Is there room for two drives in the 27" 2012 iMac?
It can be done, but I’m not sure which SKUs it applies to, or that it’s particularly advisable.
Both the 2.9 GHz and the 3.1 GHz 21.5″ iMacs have space for a second drive bay, and there’s an iFixit tutorial explaining how to install a second drive. (iFixit usually tear down new hardware within a day or so of release, and have a lot of tutorials for things like hardware upgrades and repairs).
Currently their page for the 27″ model doesn’t have such a tutorial. However, there’s a page on their forums where people are discussing associated teardowns and the possibility of such an upgrade. The poster ifixit611 writes:

The 27″ iMac holds one 3.5″ drive (unlike the 21″, which uses a 2.5″ drive), and one “blade” SSD like the new MacBook Pros use. Unlike the new 21″ iMac, you can upgrade the RAM on the 27″ w/o cutting the machine open (there’s an access hatch on the back).
Replacing the disk requires cutting open the display and removing the left speaker. You’ll need a proper mounting bracket (e.g., an Icy Dock) to mount a 2.5″ SSD in place of the 3.5″ drive. Adding/replacing the blade SSD requires pulling out the main logic board.

They conclude that they won’t be performing the upgrade before iFixit or OWC provide a proper “kit”.
So it sounds like, in theory, you could perform this upgrade on a 27″ iMac, and the instructions from iFixit for upgrading the 21″ would probably be helpful in this regard. That said, hardware is not my area of expertise and I generally get twitchy about opening up iMacs. Do at your own risk, etc.