Hi: I think we're researching the same families, so I had to respond.My sole sources at this point are census data and family stories.
My Thomas Logan was also born in SC in 1789. His wife's name was Elizabeth Owens.They moved to the Nashville area where Alexander was born in 1813, Wiley in 1823 and William (my gggrandf) in 1826.They then moved to IL where Franklin was born in 1828 and Allen (or Albert) in 1834 and Elizabeth (?)1835. I'm not sure this is them, but I've seen mentioned in an Illinois history Thomas and Alexander Logan being among the first to settle in one of the southern counties (can't find my ref at the moment)and I've found a marriage licensefor William and Elizabeth Scott in JoDaviess county on the Minnesota border where family stories say thay also lived for awhile.The 1850 census finds them in Greene County on Nov. 14, 1850, "Between Macoupin and Apple Creeks."On one page: Entries 938 (Wiley) 939 (William) 940 (an Edward Dorsey)Then I copied the page with Alexander which begins in the middle of a Flatt family, then 922 James Flatt, 293 Alexander Logan and his wife Mary Ann (a Michael Dorsey lives with them), 924 Ransom Flatt.
Some differences in info, but I'm sure this is the same family!Write me soon!