Do you still face the problem? Are you able to run opencl programs on a GPU which is not connected to display. It might be the case that you are actually running everything on the same gpu and hence things serialize.
Can you confirm that you observe the behaviour you reported with latest SDKs and Driver?
To be honest, since I had that problem I decided to move to NVIDIA considering also that NVIDIA Cuda and OpenCL do not require a running X session.
I was very disappointed also because I know that my 2 HD5770 are much powerful that my 2 GTS450.
Do you think that now my problem could be overcome ?
Yes I think executing different programs on different GPUs is available. I do not have a dual GPU system myself, so It would be nice if you can test your configuration again with latest software.
It would be nice if you can post some test case along with CLInfo output.