I have the two marriages in my database, but no links yet.
Allen TIBBITTS is in the 1851,1861,1871 and 1881 censuses at Branch Co, MI. There is no evidence from the census that he was a Rev, in 1851 he was a saw miller. (ref T9 Roll 143, page 618.
Interesting though is that his wife's name is Caroline, the second name of your Harriet. The age (both 43 in 1850) makes it possible that Harriet is their daughter - though I think we would need some other proof.