Did the Thomas marry sisters?
Thomas married Isobel Nice 21 Oct 1770 in Lerwick, Shetland, Scotland
Thomas married Goodlad Nice 15 Jan 1780 in Lerwick, Shetland, Scotland.
Thomas could be cousin of the other Thomas, as could the Nice females.
You could use credits on Scotlands People to review the marriage information which will frequently have the parent's information, as well as their ages and birth locations. Unfortunately, the marriage information does not have much more than what is seen in the index at times.