some species have been kept with conspecifics... these include bimaculoides and eldedone species but only in 500gal+ tanks, for any length of time.
In the last few years i have read about several attempts to keep octos together and not one attempt has ended well. To be honest, i wouldnt try.. not unles you have such a big tank...
... just out of sheer interest, why would you want to keep two together?