This famous video was posted to Nico Nico Douga four years ago, but was just featured on the Ota-Suke seiyuu news site.
The video is part of the lore of the supposed yuri relationship between Nabatame Hitomi and Itou Shizuka. The yuri part is probably just for show, but they are clearly close friends, with some skinship.
They are shown in an open-air hot-spring pool drinking and talking. Shizuka-sama drinks sake. I'm not sure what Naba-sama is drinking. Shizuka-sama is the first to appear in the video, and is on the right in the pool.
The video comes from their 2004 DVD Charinco de Yukou. Sometime after their first live as a singing duo, they were filmed at the Nakaya hot spring hotel at Nanasawa in Kanagawa, southwest of Tokyo.
When Shizuka-sama is leaning forward over the side of the pool to get more sake, later on in the video, Naba-sama comments what a nice pose it is, like gravure. Shizuka is known as a drinker. When the video was shot, Shizuka-sama was 23 and Naba-sama probably 27.
I think both of them are excellent voice actresses. In this current season, Naba-sama is doing a fine job as Judith in Astarotte no Omocha, and Shizuka is Chitose in Sofuteni.