Perhaps this will help: https://familysearch.org/pal:/MM9.1.1/KF7N-FYG
The birth year is different (by 2 years), but I think it is improbable that 2 different Frank Mills, both born in New York, both born in December, died 1 day apart, in the same county of Michigan, population about 33,000 at the time.
Using the above record for names, we find a sibling Anson Mills also born to Anson Mills and Eliza Darrow... living in Ashtabula Ohio, where Frank Mills married Lura Johnson: https://familysearch.org/pal:/MM9.1.1/XD4M-22T
So we have the mills family of New York in the 1840's with Father Anson, Mother Eliza, a son Anson and a son Frank. There happens to be a great matching census record at Portland, NY https://familysearch.org/pal:/MM9.1.1/MCT5-C8R