one can be the failed hollow and the second kaien... but anyway, from what i remember that hollow could eat/get inside shinigamis, so maybe he got stronger as a menos that way... but then it doesnt make any sense that rukia killed him in the past, or... if his story is true, when the reiatsu of that hollow reformed in hueco mundo it could have reformed with the reiatsu of other hollows, thus making itself a menos... and by the way his body reformed it could have been a vasto lord.