for me when I use to do it back in the 600 series days, the games i typically played (mostly first person shooters at the time) worked well.
I even dabbled around with SLI during beta of Shadowlands with two 3070s since they had added DX12U support and that included explicit multi-gpu support: https://i.imgur.com/sF35HdX.jpg
It worked! my fps shot up from ~70fps with a single 3070 to ~120 with two.
I did a post talking about it here: Explicit Multi-GPU Support added to Shadowlands!
unfortunately Blizzard shattered the dream of sli for the retail launch, as blizzard didn’t carry over support for it into the final launch :\ however by then I didn’t care too much as I had acquired a 3090, so no need for two gpus anymore.