When the first half of the city is ready, the next step is to use duplicate to turn it into a complete 4-player map. Even after you’ve duplicated the first half, you can still keep making changes to the city.
In the Model tab, collisions should be off (not highlighted). Collisions will make the map difficult to move and rotate.
Click and drag your mouse to select all the buildings, roads, and props.
Duplicate the selection just like you have before.
Use the Move and Rotate tools to reposition your city. If it doesn't look right, just undo (Ctrl + Z or ⌘ + Z) and try again.
After duplicating and rotating the city, you can add or adjust tiles to make the halves fit together better.