Примеры предложений: 

No less than 80%  of the sites are concentrated near to the confluence of the Amur and Ussuri rivers. 

For example, the Xiaonanshan and Taoshan  sites located along the southern edge of the Middle Amur depression may be associated with the Osipovka culture. 

The Gromatukha area tied to the confluence of Zeya  and Amur rivers    is even larger and stretches up to 1000 km along the mainstream of the Amur   River and 700 km along the Zeya River.

нативные коллеги говаривали, что повтор артикля делает стиль stilted

Correct. It's for style reasons -- to avoid repeating the word 'the', because it's understood each time, and so 2nd and subsequent 'the's are considered superfluous:

the Amur and ['the' understood] Ussuri rivers

the Xiaonanshan and ['the' understood] Taoshan sites

** повтор артикля делает стиль stilted **. Exactly.

Similarly,

* the Labour and Conservative parties;

* the Chinese and Japanese governments.

(Incidentally, "the confluence of Zeya and Amur rivers" should read "the confluence of the Zeya and Amur rivers". The two other примеры you've given are correct, though).

Да, в 3 примере пропустила артикль, спасибо!