I found the following from http://www.snbforums.com/threads/how-to-setup-2-rt-ac66u-routers.10384/#post-158096
"Not to dredge up an old thread, but in order to provide an update and clarify for anyone looking to use two AC66U's, the RT-AC66 does now allow "Repeater" mode, and both the router...
I have bought an ASUS RT-AC66U recently.
I've tried the stock firmware (after a firmware upgrade) as well as asuswrt-merlin.
I've read at some places, e.g. http://www.tomshardware.co.uk/answers/id-1960218/media-bridge-question-asus-ac66r.html that the router supports getting a 5GHz Signal from...