I have no experience on Mac and moreover I'm not an expert in dual monitor too. But I want to do some brain storming, hope this help.
First of all, if your monitor supports hardware calibration ok, but if calibration is by software then videocard LUT are changed by the process.
If your system has only one video card and software calibration is performed, there is no way to calibrate the second monitor.
I don't think PS can manage colors for 2 monitor, I think this could be possible at operative system level but not at application system level.
Application system level does not converse directly with videocard, it send data to operative system that send data to videocard.
So an application can't send 2 flow of data, telling to the operative system wich one is for monitor1 and wich one is for monitor2.