Ah, I had to change the mainboard that supports 4 pcie 16x slots.
Then I put the cards to maximize space between them: [device0] [empty] [device1] [empty].
Then it seems to be ok.
When I installed like [device0][device1], the device0 became too hot (within 5 minutes) and throttled down.
It was ok when I used 1080. Similar slow-down for 1080ti (although I was not aware because it was not as clear and frequent as 2080ti).