iMac 27 (Late 2013) with two Thunderbolt displays The iMac 27" late 2013 (Haswell processor) has 2 IO Thunderbolt ports. Is it possible to run two 2560x1440 displays at full resolution (Thunderbolt or via Dual DVI adapter) on the side of it's main display?

As far as I know, the previous iMac could handle them with no problems and MacBook Pro Retina can handle them as well.
Yes its is possible. You can daisy chain (connect) two thunderbolt displays. 
See http://support.apple.com/kb/HT5219#18