Although Octane can use multiple graphics cards it only uses the memory of lowest memory card because all the graphic cards have to load into their memory. So if you got a 4Gb and 2Gb card each will use 2Gb. Pity it does not add together but there you are. So mostly you are better off with one good card.

This is no longer true in the latest version of the octane render (2.2). There are no more limits on the number of textures or the total size of textures. It will offload them to CPU RAM if needed. But it will cost performance