why used double graphic to render snapshot more noise then single one ?

[sheng_peng]

With the same parameter, the noise of two graphics cards is more than that of a single one

[vladimir.nedev]

The light cache in Vantage doesn't support multiple GPUs yet.
So when you use both GPUs, the light cache is disabled which leads to more GI noise.

Greetings,
Vladimir Nedev

[vladimir.nedev]

What I mentioned above is the most likely reason. Without the scene to test it's only a guess.

Greetings,
Vladimir Nedev