For some older Mac OS X versions 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 is recommended to reboot the system. Then re-open Agisoft Metashape Preferences window and check on your NVIDIA cards in the GPU tab.
For the latest Mac OS X versions (starting from macOS 10.14 Mojave) where CUDA is not supported, Agisoft Metashape will use OpenCL implementation automatically for GPU-based processing on NVIDIA graphic cards.