I`m using Intel Nuc8 Hades Canyon HVK with intermal RX Vega M GH
and I want to combine it with eGPU like RX Vega 64 or other.
Does Crossfire combine power of both GPUs or not?
No, crossfire only works with 2 of the same series cards, aside from a few mobile exceptions a few generations back. I don't think it would work for you. In fact I'm pretty sure it isn't supported on external gpus. If you want to know for sure and get an official answer you can ask AMD support: https://www.amd.com/en/support/contact-email-form