If you manage to find 2 used 7900XTXs for less than one R9700 then yea, go for it.
VRAM is more important than memory bandwidth though, and please research about "combing" two GPUs for inference. I'm not talking from experience, but I heard it's not as simple as it seems (especially for Radeons, nVidia has nvlink, the last time I heard about crossfire was 2010). One 36GB VRAM may end up being better than 2x24GB.
