New York Center is using frequency cross-coupling on their Oceanic Sectors. ZNY Sector 82 on 126.025 is cross-coupled with Sector 83 on 133.525, Sector 86 on 133.500 cross couples with Sector 65 on 125.925, when it's slow I've heard all 4 sectors cross-coupled together. (hint: this would make an interesting feed if anyone is in range of any of these ground stations). I've also heard cross-coupling done on some Cleveland Center low sectors, someone mentioned here on this forum in another thread that it was being tested there. So far these are the only 2 areas in the US I've heard it used.
Cross-coupling is used extensively in Canada, for those interested, a good example of this can be heard on Montreal Center on the YUL ACC (Noranda Sector) 133.975 feed here on LiveATC. During normal hours this controller is cross-coupled to around 5 frequencies and during the overnight hours up to around 18 frequencies are linked together. It's easy to tell when an aircraft is transmitting on a cross-coupled frequency as there is a distinct sort of tinny sound to their transmissions and always the same strength as the controller.