For Mac OS X it might be necessary to install additional CUDA drivers pack to enable NVIDIA cards utilization. Drivers should be downloaded and installed from the following web-page:
After that it may be required to reboot OS. Then re-open Agisoft Metashape Preferences window and check on your NVIDIA cards in the GPU tab.
For some latest Mac OS X versions CUDA is not supported, instead Agisoft Metashape will use OpenCL for GPU-based processing.