Best Use of a EGPU with FCP

I have an iMac Intel Retina 5K, 27 inch, 2020, 3.8 GHz 8-Core Intel Core i7, 64 GB Ram, Graphics AMD Radeon Pro 5700 XT 16 GB.  I also have connected to it a Sonnet EGPU Breakaway 750 ex box with an AMD Radeon 5700 XT 8GB card.  My question is what is the best way to make use of these GPUs in FCP?  Considering the locations and specs on each set up will one be better to use for performance than the other?  Is it possible to use both?  Please advise what you would suggest.

The internal 5700XT / 16 Gb GPU has more ram than Sonnet eGPU and is directly connected to motherboard (faster speed data than ThB3 eGPU), so IMO I think you will get better performance with the GPU installed in the iMAc. What could you do with eGPU? Keep in mind that FCP and Motion don't use GPU's simultaneously, FCP allow you chose a GPU for render/share but Is a good choice for Macs with less GPU powerful and have installed an eGPU.

Thanks for your help!