Why did the twins take that action? Because of the 200000 Celestial Points reward offered by the system. It wasn't as easy as Rean said it was. To get the Big Five to have people following everyone, they had to convince their leaders again to take that step further.
200000 Celestial Points were directly linked to the difficulty of the task. Roan was pretty certain that just giving Zuco half of the reward wasn't going to be enough to get the turtle. Roan was right, since Zuco indeed had no intention of doing so. Thus, that plan, which worked in the end... even if they had to spend a few of the Celestial Energy Stones for it.
Had they not taken that precaution, chances were they wouldn't ever retrieve Hyeoumu's body, at least not without a lot more effort. Too bad that Roan was still not convinced. All these obstacles didn't seem to be enough for that reward.
