In this case both cities are in the same state, so there is no point trying to distinguish them by using the state as a byname. But there are other cases that use rivers or other geographical features although they would be distinguishable by state.
The most famous one in the USA may be Rothenburg ob der Tauber (Rothenburg above the river Tauber, Bavaria), which has to be distinguished from Rothenburg (Saale) (Rothenburg at the river Saale, Saxony-Anhalt) and Rothenburg/Oberlausitz (Rothenburg in the Upper Lusatia region of Saxony).
In addition, there are more places called Rotenburg without an h. Rotenburg (Wümme) at the eponymous river used to be called Rotenburg in Hannover after the eponymous kingdom until the government of Lower Saxony discovered in 1969 that the kingdom of Hanover did no longer exist since 1866. Rotenburg an der Fulda is also named after the river.
The reason why so many places are named after rivers is that they were founded at historic river crossings, first fords, later bridges. At these times, the places had no distinguishing byname as most people were not traveling. But with the expansion of trade, they were named after the most important feature for traders, the river crossing. And the reason why there are so many Rot(h)enburgs is because the castles protecting the river crossings were either made from red brick or from red sandstone.
It gets even more insane with Neustadt, New Town. These places were often founded long after the medieval period by the respective monarchs of the area, often to accomodate immigrant refugees. Here is a list of Neustadts:
Neustadt am Rübenberge historically Neustadt am Rouvenberge after a flat, rough (= stone-covered) elevation in the otherwise flat landscape
Neustadt an der Weinstraße after the wine-growing region in Rhineland-Palatinate
Neustadt in Holstein after the Duchy of Holstein in today's state of Schleswig-Holstein
Neustadt bei Coburg near the bigger city of Coburg in Bavaria
Bad Neustadt an der Saale river, the prefix Bad indicates that it is a state-recognized spa
Neustadt in Sachsen after the Kingdom of Saxony, largely identical with today's state of Saxony
Neustadt an der Donau (river, Bavaria)
Neustadt an der Aisch (river, Bavaria)
Neustadt (Hessen) (state)
Neustadt an der Orla (creek, Thuringia)
Neustadt (Wied) (river, North Rhine-Westphalia)
Neustadt an der Waldnaab (river, Bavaria)
Neustadt am Kulm (mountain, Bavaria, belongs to the district of Neustadt an der Waldnaab)
Neustadt (Dosse) (river, Brandenburg)
Neustadt am Main (river, Bavaria)
Neustadt/Vogtl. (historic region of the Vogtland, today straddling the borders of Saxony, Bavaria, and the Czech Republic)
Neustadt/Westerwald (forested hills in Rhineland-Palatinate)
Neustadt unter der Harzburg (after the medieval castle, now part of Bad Harzburg)
Neustadt am Rennsteig (ancient roadway over the heights of the Thuringian Forest, Thuringia)
Neustadt/Harz (mountain range, Thuringia)
Neustadt bei Hachmühlen (neighboring place, Lower Saxony)
Neustadt/Spree (river, Saxony)
Neustadt im Schwarzwald (Black Forest region, today part of the double-city Titisee-Neustadt, Baden-Wurttemberg)
Neustadt an der Rems (river, today part of the double-town of Waiblingen-Neustadt, Baden-Wurttemberg)
There are even more Neustadts not mentioned here because they are villages, located in Austria or Switzerland, or in formerly German territories.
You see that there are often multiple Neustadts in a state, so there is no point naming them after the state. There is also no rule whether to use brackets, a slash, write the byname out or abbreviate it. This has grown historically, and you have to deal with it.
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