Kill the Twins
The Twins are located at the Palai Ferry Station. The first Twin you will encounter shortly after getting out the tunnels and reaching the beach. The second Twin is located in a storage warehouse, to the right of the ferry station. To get accesses inside you will need to climb up the billboard and pick up all the items inside the truck bed. After doing so, the doors to the storage warehouse will open and a few zombies will come out and also the second Twin will be inside.
Now if the second Twin is not in the storage warehouse the first time. Wait a few hours or complete a quest or two, and then come back to the storage warehouse and try it again.