No it will only use one graphic's card vram however speed is combined. Also, something to keep in mind is that if you have two different GPUs running in one system, Octane will use the GPU with lowest ram available. For example you're running a gtx 460 2GB and a 480 1.5 GB whilst running octane and ...

i seen the film "Real-time rendering in Octane render with 2x GTX480 " on youtube,
i got question about render speed,
2 GTX480 video cards = double render speed、double video memory?
can anyone tell me?

sorry my english is very poor...