Help with multiple gpus

Last week I finally got a rtx 3070 which I changed out for my gtx 1080ti. As I plan on building a gpu mining rig in the coming weeks, I got myself some pcie risers 009s Plus. I first tried out inserting both gpus in the pcie x16 slots which worked fine but due to the minimal space in the case, they got hot fast. I left my 3070 in the primary pcie x16 slot and tried using one of the risers I got instead. Sometimes the 1080ti on the riser (pcie x1) would not even be detected. When it did get "detected", my pc would either freeze, restart or not boot up at all. By detected I mean appearing in device manager but not in task manager (afterburner would detect a second gpu but not show the model, clocks, nothing). I tried reinstalling the nvidia driver but with no difference (even DDU), reinstalled motherboard drivers. Even got BSOD one time with MEMORY_MANAGEMENT error and another time with SYSTEM_THREAD_EXCEPTION_NOT_HANDLED (nvlddmkm.sys).

Any suggestion / help is appreciated. Thanks! (**This is my first reddit post ever**)

CPU: AMD Ryzen 9 3950x

MOBO: MSI B550 Tomahawk

RAM: 2x 16GB Kingstone DDR4-3200MHz

Primary GPU: Asus rtx 3070

Secondary GPU (the not working one): Asus gtx 1080ti

PSU: Corsair HX850