Le Parisien: Des étudiants FN évoquent les cours d'orthographe... avec une faute
The Conversation: Dictée guidée : un nouveau moyen de progresser en orthographe ?
However, the split took 2 cours, from 29.03.2008 to 05.10.2008. If not for the 3 months condition, I'd say that this was indeed first series to air in split-cour format.
The most popular hypothesis is that it derives from French cours, cognate to "course" as in "lecture". 3 Note that the English "cour" is effectively a back-formation from cours (which is singular in French), and the singular/plural distinction between "cour" and "cours" is an English innovation.
So a 26 episode series can be referred to as "2 cours". But as far as the length of an anime "season", a 12 episode series run can be referred to as a "season", yet sometimes a 26, or 36, or 201 (Gintama) episode run is called a "season". It's technically not tied to a specific length, which is why some people prefer to use "cours".
Here is one clear reason this would happen: Long-running cash cows aside, the number of cours in a particular season of a particular anime is a small integer - almost always 1, 2, 3, or 4; and most frequently just 1 or 2. Even if anime season lengths were randomly either 1 or 2 cours, we'd expect to see 50% of two-season anime be "symmetrical".
Anime releases are also split into cours, 13-week blocks of episodes also sometimes referred to as "seasons". (As described in What is a "cour"? and What defines one "season" of anime?.) Some shows, such as Madoka, Bakemonogatari, and Ore Imo, run one cour in length.