How does OSX handle multiple GPUs?

Hello everyone,

Been trying to find a answer for this but after some googling I have not been able to find anything in detail. I was wondering how OSX handles multiple GPUs, being since OSX does not support SLI or Crossfire? Does OSX some how pool the resources of both cards together? Or does it simply keep the two cards separate and they do not share resource with on another?

Thanks!