Dividing could be possible if you were crafty, but even if you had that in place you have to consider --
1) You can't have corals with him or he will disrupt them or be hurt by them
2) Even if his area is okay, the temperature he wants is NOT going to be what the corals want -- or what you'll be able to provide with strong lighting. I'm assuming you would get an o. bimac, these are colder water species and the colder the better. Getting up past 76F'ish (or lower depending on who you ask) is going to be stressful to the creature and will shorten his lifespan.
3) Logistically, doing this separation will be very difficult and probably even expensive.
Unfortunately, you would be considerably better off giving the octopus his own system... I think if you wanted the 90 to be a F/O tank it would be possible, but still very tricky.
Good luck
Robert
