The fact that they are two cities means that there is a perfect conurbation that caused the clash. Something similar happens in Caracas with The Guaira and Miranda. If I am wrong, they form a large megalopolis, although it is not officially recognized.
According to what my brother has told me, the capital is from Bolivia, it is nice unlike the towns on the border extremes. I love that you are telling us about your experience in the country. :P