Some years back, the Monterey Aquarium O. Rubescens exhibit had several small octos in it together. I have no idea if that was an experiment that didn't work out, though.
I was greatly amused to see one octo swim up to a large empty barnacle shell, reach a few arms in, and evict the other octo that was hiding in it, then go in to claim it for itself. I seem to recall that there was a third octo hiding in a corner when all this was going on.
However, as others have said, the anecdotal evidence is that more often than not one of the tankmates ends up as lunch for the other. Also, there is some evidence that the octos are uncomfortable all the time, maybe because they always have to be on guard in case the other one tries to eat them. Remember, octos are pretty much solitary in the wild, although being eaten by breatheren seems to occur with the social cephs like squids and cuttles a lot, too.