IF you are using windows, may I suggest using MSI Afterburner for adjusting clocks and fanspeeds? I know it's handy with CGMiner to have it in the config and just have the clocks apllied when CGMiner starts, but I personally find MSI Afterburner a lot more comfortable and I like it's HWMonitor that keeps me updatet on the core temps.
Even that won't work.
I had both cards running on my main-pc before setting up the mining rig.
Overclocked them with Afterburner and tried to downclock memoryclock as well...but there was the same problem as mentioned.
Wasn't able to go lower than 1050Mhz...