The Optix "core" by itself does support SLI (and SLI through NVLink), as it's core nVidia product :- ). If the Optix Denoiser itself take advantage of it, I don't know but I think it hardly matters because the Optix denoiser is very lightweight and doesn't take much performance at all.
Regardless of what support GPU you have, it will still be miliseconds. It will take more for Corona framebuffer to refresh that it will take for denoiser to apply.
You can't specify though for each GPU to do something else because Corona doesn't support such differentiation. I.e GPU 1 for viewport, and GPU 2 for computation (Optix denoiser). Only GPU and mixed renderers have such options.