How confident are you that it's using AFR?
Also, it seems to me that, even if it is using AFR, the downside of that extra frame of latency would be a lot less at higher framerates. Ideally, if the CPU is just twiddling its thumbs, waiting on 1 GPU to display so it can start rendering the next frame - with AFR, it can fill that idle time by queuing a frame to render on the second GPU. That simplistic description presumes no double-buffering, but you get the idea.
While there was talk of split screen rendering for multi-GPU, to my knowledge that was almost never implemented. It required games to directly code for multi-GPU, rather than having the drivers doing the work. Way back in the early SLI days, there were some examples of SFR, but I suspect nothing since 2015 has done multi-GPU without using AFR. And of course, 3DMark was never a game engine and thus had no real reason to look at other approaches. SFR basically requires more work and doesn't scale as easily as AFR is my understanding. But, LOL, some tool did write this back in the day: https://www.anandtech.com/show/8643...crossfire-with-mantle-sfr-not-actually-broken
Anyway, with AFR my understanding is that to avoid having the game potentially get ahead of itself, you basically render frame 1, render frame 2, send frame 1 to the actual display, render frame 3, send frame 2 to the display, render frame 4, etc. Maybe some games were better about this, but I'm pretty sure they always showed frame n-2 whereas with one GPU you'd show frame n. If I still had two of the same GPU with the requisite NVLink or HB-SLI bridge, I could test! (I probably do have the necessary parts, but some are buried in an unknown box.)