Initially, this was a good idea because it gave Jack something to do and it also gave him a sense of authority. Jack might have rebelled against Ralph sooner if he (Jack) had no prominent role to play. By sharing the responsibilities of being leaders, Ralph reached out to Jack and this gesture implied that they were both leaders.
Jack and Ralph smiled at each other with shy liking. The rest began to talk eagerly. (Chapter 1)
They are both natural leaders. It's just that Jack is more interested in power and Ralph is more interested in survival.
You could also argue that, in the end, giving Jack any responsibility was a bad move because once he got a taste of it, he just wanted more and more. A better option might have been to make Jack a leader in constructing the huts, therefore giving Jack something more productive to do: something that would be less likely to encourage the kind of animal-like behavior that he associated with hunting. But even in this case, Jack might have been inclined (sooner) to forgo the huts and go out hunting because that's where the action is.
While there are good and bad points to making Jack the hunter, it seems that it was essentially a good idea because it kept Jack feeling like he had some power of authority. And it kept Ralph in charge of the more important things such as shelter and keeping the fire going. Also, in Ralph's mind this was all temporary. He might have guessed that Jack would eventually get out of hand. But in the meantime, splitting up the responsibilities would keep Jack happy. This was all temporary to Ralph because it was all as a means to survive until they would be rescued. The rescue just didn't come before Jack started on a power trip.