My rig includes a GTX1070 8Gb gfx card, and as much as possible I render on GPU only.
Against the odds these days, I have a 2nd GTX1070 8Gb gfx card on the way (and I know, they are like hen’s teeth).
I bought this almost exclusively for Blender, but will probably also use the 2nd GPU as a passthrough for VirtualBox. The latter is irrelevant.
In Blender, can I expect a halving of rendering time by rendering on two GPUs?
EDIT - I know that the preparation (building bvh etc.) will not be improved. It is once the render actually starts that I’m interested in.