My 3080 takes up 2.5 slots which basically is 3 slots, paired with the RX 570, it leaves little room to breath. Idle temps of both GPU’s were the mid 50s. The 3080 on its own with airflow fixed now idling at mid 40s. My case is NZXT H710i, there wasn’t enough breathing room. However if one of the GPU is mounted vertically, I think airflow will get better than my setup.
I ended up installing 2x 120mm fans just beneath the GPU and converted my front radiator to a push/pull configuration. Together with all the screens connected through DisplayPort, fps didn’t suffer much.