Glad to see you back.
(see, all you naysayers, he wasn't just trying to get a rise out of us)
it's not really so unusual that they haven't killed your fish, it's more of a concern that there's nothing much stopping them from killing them, so there's certainly no guarantee that your fish will survive... also, fish sometimes stress out octos, so it's usually not recommended to have both together, since the fish often are killed, or the fish stress out the octos and they hide all the time or have health complications.
Although it's possible that what you saw was mating behavior, multiple octos in the same tank do sometimes fight with each other, sometimes in mild, semi-friendly interactions and sometimes in more "fight to the death" disagreements.
I don't know if you read my extended responses above, but what you were told about "they don't like air" is not really accurate, and certainly not guaranteed. Octo-proofing your tank would be a very good idea, and making sure that you know that there could be a dying (and hence maybe extra-aggressive) octo *outside* the tank when you least expect it is really a necessity, since we've seen octos escape from "completely sealed" tanks pretty often. Having another octo and fish actually may make it more likely one will try to escape: octos can be territorial, and in the wild, they might decide that if there's another octo or a bothersome fish in their tide pool that they will go exploring to find a home with less competition for food. Similarly, octos are more likely to choose to leave the tank if the water quality is bad, like if your filtration fails or there is a partially-eaten crab fouling the water... normally, we recommend removing uneaten food quickly because of that, but, of course, make sure not to risk a bite while recovering crab bits.
. Strangely, 
and 
were included right away on the first request 
and another I have been playing with.
