After you've gotten the one Chaos Crystal & given it to Hope you will have the ability to obtain a Odin Blade for either Serah or Noel. To get a Second Chaos weapon for the other person go to Serendipity & talk to the rewards lady. You will need 750 coins. At the very bottom of the list is a second Chaos weapon with the exact same stat's as the first, this way both Serah & noel can both have Chaos weapons that will level up with every fragment you acquire.