So... perform renders with the following environment variables set.
This will test with only the first GPU enabled
KARMA_XPU_DISABLE_DEVICE_0=0 KARMA_XPU_DISABLE_DEVICE_1=1 KARMA_XPU_DISABLE_DEVICE_2=1
This will test with only the second GPU enabled
KARMA_XPU_DISABLE_DEVICE_0=1 KARMA_XPU_DISABLE_DEVICE_1=0 KARMA_XPU_DISABLE_DEVICE_2=1
This will test with both of the GPUs enabled.
KARMA_XPU_DISABLE_DEVICE_0=0 KARMA_XPU_DISABLE_DEVICE_1=0 KARMA_XPU_DISABLE_DEVICE_2=1
more instructions here
https://www.sidefx.com/docs/houdini/solaris/karma_xpu.html#disablingdevices [www.sidefx.com]
I'm guessing you'll find the first two have the same approximate time, and the last will be approximately twice as fast. Please note that its best to run the tests twice, to ensure all GPU shaders are compiled and cached.