Simple concatenation of two names (whether hyphenated or not) does not produce a portmanteau.
During the Joseon Kingdom, seven Korean provinces (all but the region around the capital) were named by combining the first characters of their two major cities.
Or they're a blend of county names that the district covers parts of.
A border portmanteau combines the names of two, or occasionally three, adjacent polities (countries, states, provinces, counties, cities) to form a name for a region, town, body of water, or other feature on or near their mutual border.
[9] Note: places listed on the same line are immediately across the border from each other.
Lakes that are on or near borders also sometimes get named with portmanteaus of the neighbouring polities.
Also included are pseudo-border portmanteau towns (yellow dots).