For some older macOS 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 macOS 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.