Kathie, thanks for the comment. I am fairly certain it is the same woman. The only record found for her is a deed on 23 Dec 1778 between Joseph Hubbard and Frances his wife, late widow and relict of John Cobb deceased releasing all their rights to her dower in the land of her late husband John Cobb deceased. They aren't actually selling the land, only the dower interest.
There is no source for the marriage between Joseph and Frances, and I believe the date of 1739 was assigned to this second marriage simply because Joseph had children born in the 1740s. I also believe Joseph was married previously as well and the children are all from that first marriage.
I just wanted to see if there might be a case which would make the date of 1739 for the marriage make sense. It seems as though you agree it probably didn't happen that way.
.edited to correct date of deed