Awesome, looks like this was found out in the last year. I remember back in 2018, it was you cannot have 2 cards in there.
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Once again - a warning - I am very very far from an expert.
From my reading a couple of years ago, Apple had an intention for two GPUs. The later 6,1 featured two after all. Some videographers made substantial claims about how well two RX 480 would operate Final Cut. The 480's having an advantage in being narrow and reasonably well performed (much the same available as an RX 580 if one updated them). Power supply issues of course with two cards ...