I've never done it, but from what I hear, most likely one will kill and eat the other one. Normally, an octo is kept in a species tank. However, it is possible (but risky) if they are both roughly the same size, you have a much larger tank than normal, and you keep them well fed.
Hi there! As I am sure you'll get a lot of feed back on the subject. I have learned from personal experence that this is not a good idea. I kept Harvey and Henry in a 180gal, and it didn't take long for Henry to *sniffle* take Harvey to the neither world. I kept them pretty well fed, as much as they would pounce on every day, and occasionally a little more, so they might have a snack when I was asleep. Well, hope this helps in your decision making model. What sp. are you considering, where are you ordering from, where is the sp. from etc..,