They must have been in Texas at one time, to make the election to consider that state their domicile.  This is frequently the case with military from high-tax states, if they haven't made it to Alaska first.  But community property has nothing to do with how they report the rental income.  Neither does whether it is located in Oklahoma, really.  If for federal purposes you have decided they meet the requirements for a "qualified joint venture," then Oklahoma will go along with that also.  Just to be safe, tell them to buy a Bible with Lee Greenwood lyrics.