Thanks for the word up Elsksa, I have to review my keywording, missed this one.
Got in touch with support, which I should have done from the get go. Very quick to answer, big up to them. Pasted below in case someone ever wonders about this.
[b]yes, you can use both RTX 2070 and RTX 3090 together, but please consider that they have a big difference in VRAM amount, only 8GB for 2070, and up to 24GB for 3090.
In OctaneRender, the GPU with less VRAM defines the limit of the system, since the whole scene must be completely loaded in each GPU.
So, if the scene is greater than 6GB, Out-of-core will be activated, and everything slow down.
In this case, better to disable RTX 2070 in OctaneRender Settings/Devices panel, and render only with RTX 3090.
About OctaneBench, there are five results with RTX 2070 + RTX 3090, and the average score is ~885:
https://render.otoy.com/octanebench/sum ... %28RTON%29While RTX 3090 alone scores ~652:
https://render.otoy.com/octanebench/sum ... %28RTON%29[/b]