I recently purchased 2 RX 570 for mining/gaming.
Both of the cards successfully mine at stable temps and give high hashrates. They also have a modified bios.
When switching to gaming, enabling crossfire and "gaming" instead of compute. Only one GPU has utilization while gaming.
Tried running DDU/new Drivers/Patch and it still doesn't work.
My motherboard boasts capability with crossfire.
So, you do know that not all games are coded for crossfire/sli, check on the game/s support/forum.