Could not find anything interesting in kernlog, syslog or dmesg. But what I did find was this github issue 14 where people have a similar problem. GPU burn etc cannot make the system crash and PSU is powerful enough (on paper). They say, that PyTorch causes a short big power surge which can cause the PSU to fail, even if it’s big enough to support the system on heavy load in other circumstances (gpu burn etc).
I put the 2 2080Tis into a system with a 1600w PSU and they seemed to work fine. The PSU in the other system had 850w. Obv. I cannot guarantee that this is the cause/solution due to many variables being changed but as other people have had similar experiences, I will take it for now (and will post again here if there are new developments). Thanks for your help!
(and btw, since many people have the same problem which seems to originate in PyTorch, I think it would be good if the relevant developers could have a look into that)