There was a lot of speculation that he had a partner for at least some of his offences, I believe it was mooted that his brother in law was a possibility.
This was based on some of the very early known attacks before murder itself became the main objective (there are still some murders on the books from this period which are very likely to have been carried out by Sutcliffe but murder itself does not seem to have been either the main reason, or the at least as ruthlessly carried out)
Those earlier crimes did not have the mutilation aspect, but there is a pattern of escalation.
One of those early attacks was carried out on a woman walking near a couple of railway bridges at Wortley in Leeds.
She says she was passed by two men walking the other direction one of whom she believes attacked her with a hammer and causing serious head injury.
Her attacker suddenly broke off and left, but that second person has not been identified.
I know that particular area well enough to not want to be there, there are at least two railway bridges and a canal aqueduct crossing closely at angles over a road which is effectively part cut into a banking.The place is ideal for an attack, no-one goes there on foot and very little traffic goes along it, being poorly lit at night and not much of a route to anywhere. It is the sort of place where you would notice another person or persons walking along, I do not doubt that she saw two men, and she would remember their presence as it would have been reasonably noteworthy.
This crime itself has not been proven against Sutcliffe but the M.O was strikingly similar to later ones.
There has been speculation that non-lethal violent attacks by Sutcliffe may date as far back as 1970, at any rate police have been looking at unsolved offfences going back that far.
I don’t think anyone believes that Sutcliffe was not responsible for the crimes he was convicted of, and if there was another offender working with him one would have expected police to find some link when his house and garage and workplace were searched.
One murder in particular gives some indication of how he wanted to take the “credit” for his crimes.
He killed Yvonne Pearson some time before one of his other victims, but a full two months passed before returned and dragged the body to a postion where it could be found more easily.
It would seem to me that he wanted to increase his notoriety, and from this it doesn’t seem like he is the sort of person who would want to “share” in this.
If someone did work with him, I doubt that it was for long, he was seen by police a goodly number of times in red-light areas, but not with another person, his surviving victims that were definately attributed to him have not mentioned another person.
There could have been other killers, completely independant, after all Fred West carried out his crimes completely undetected for much of this period as did Robert Black, but someone would have to come up with very convincing evidence of anything more than that.