I stumbled upon OpenCL Device in the preferences window, and it implies that only 1 gpu at a time will be used for openCL. TBH I don't see a lot of strain being put on my first gpu, but does this mean houdini can't use both gpu's for openCL calcs?

Thanks

That's correct. But you can use 1 gpu for graphics and 1 for opencl, which is the recommended way to use 2 gpus.

What about sli?

If I had 2 gpus running in sli would Houdini be able to use both gpu's then?