The ichinomiya of Yamashiro Province

What the Japanese sources say

The Kyoto Prefecture government says of Kamowakeikazuchi Jinja (賀茂別雷神社, also known as Kamigamo) that it “became the ichinomiya of Yamashiro Province from the Heian period onward, together with Kamomioya Jinja (賀茂御祖神社, also known as Shimogamo Jinja).” It is written as the two shrines holding the title together, not one or the other.

The two shrines were originally one Kamo shrine, the clan deity (ujigami) of the Kamo family, which was divided into an upper and a lower shrine, and the deities they enshrine are related to each other. Tamayorihime no Mikoto, enshrined at Shimogamo, is the mother of Kamowakeikazuchi no Okami, enshrined at Kamigamo. The upper shrine stands on the upper reaches of the Kamo River and the lower shrine downstream, and the Aoi Matsuri (葵祭) is held by the two shrines together.

Neither the Kyoto Prefecture site nor the official sites of the two shrines says when the shrine was divided in two, or how, so this could not be confirmed.

Written from Japanese sources, each quoted. The Japanese page has the same discussion with the original wording.