You're right, there's almost no way to prevent the joining of the two city parts.
I would habve made another design: some tiles with 1/3 left and 2/3 right and some with 2/3 right and 1/3 left. Then you could join the cities parts with most of the city tiles but - with luck and a mirrored tile - could separate the two cities.
One of my friends further suggests that we make a series of similar tiles, the divide of the city segments occurring at 0.2, 0.4, 0.5, 0.6, and 0.8 times the tile length from the left. Therefore, there are multiple groups of mirror images.