Hi pobedeen,
yes, if you add one or two GPUs via Thunderbolt 3, you can render with 2x/3x GPUs at the same time.
OctaneX scales linearly, and the boost depends on the AMD GPUs you choose:
Please note that also VRAM amount is important.
The GPU with less VRAM defines the limit of the system, since the whole scene must be loaded in each GPU.
So, if your internal GPU has 8GB of VRAM, and you add a GPU with 16GB, the internal GPU must be disabled, to be able to render with more than 8GB of VRAM.
While, if the scene stays in 8GB of VRAM, you can render with both GPUs at the same time.
Happy Mac GPU rendering,
ciao Beppe