Have you seen this webpage? There's a serious lack of information about siblings for these two people. Various Ancestry.com members list ten or more siblings but the records for these siblings indicate they were born in 1840 or later. The records don't match.
I was about to forget We had a Michael here in an adjoining county and he
often visited my father and family. His father was Lebbeus Burtz, who was a
son of Levi Burtz, the latter a brother to grandfather Joshua Burtz. Levi
Burtz, who wrote you was a brother of Lebbeus Burtz. I know of several
Levi's, some of them sons of daughters, and a number of Frederick's, to-wit,
Uncle Frederick Burtz, his son Frederick, my brother's son Frederick, and
Michael just mentioned had a son Frederick who died a few years ago. I have
already named the Mike's, four in number.