R' Ezekiel (Chaskiel) Landau

http://historyofgenealogy.blogspot.in/2015/11/the-landau-monologues...

In addition to the above link, there is also a footnote in the 3rd edition of Unbroken Chain, in Chapter 7, page 155 indicating it is likely that Ezekiel was married twice, with the wives being first cousins. The main text specifies the two wives as 1) Nesche, born about 1804, and 2) the daughter of R. Joseph Samuel Landau, ABD Kempen.

Regarding Ezekiel's father Isaac, a footnote at the bottom of the page states the following:

"R. Joseph Lowenstein in an unpublished manuscript (a copy in posession of the present author) states he was married twice and had issue with his second wife, connecting their son Jacob to the Friedensohn family."

It appears that Jacob (G12.1) was the first born son of Isaac, and that Jacob and Ezekiel (G12.2) were both born to the 2nd wife. There is no mention of Isaac having any issue with his first wife.