I am having exactly the same problem - the two screens frequently do not wake up after a computer sleep or shutdown. I have to power off and on at least once. I am using the included DP to mDP cables connected to a late 2013 Mac Pro running OS X 10.9.3. DP 1.2 is enabled. Is there any update on this issue - it is a real pain.
We do not test on apple hardware, so how can there be an update? DELL-Chris M