There is a section on sequencing in YinSights (
http://www.yinyoga.com/ys2_2.2.4.0_sequencing.php ) and the topic has been discussed several times on this forum. (See these posts, for example:
http://www.yinyoga.com/forums/viewtopic.php?t=90 or
http://www.yinyoga.com/forums/viewtopic.php?t=398) You may want to read these. However, your questions about how often to work the organs specifically has never been raised before.
From my own experience, you can mix and match which organs you are targeting, but generally, I would do something for the Kidneys in every practice. The Kidneys are the home of Jing, which supports and nourishes all the other organs. Backbends are good ways to work the Kidneys: Sphinx or Swan will do it. There are several flows in YinSights that target specific organs, including the Kidneys. (
http://www.yinyoga.com/ys2_2.2.6.4_flow_for_kidneys.php)
In Yin Yoga we generally target the lower body, and thus we work the lower body meridians and thus their pertaining organs. (See YinSights again for the meridians and organs:
http://www.yinyoga.com/ys1_3.2.5.0_the_meridians.php ) This means we predominantly work the Liver/Gall Bladder, the Kidneys/Urinary Bladder and the Spleen/Stomach. I usually have one of these pairs as my main target area when I do an organ-specific practice, but again, I will always work the Kidneys too.
Hope this helps.
Cheers
Bernie