I understand what you are trying to do, but as the ages are so close, it will be difficult to distiguish thetwo without a parent's will.
It seemed that possibly Sarah Shimmin and Thomas Moore and family had left the Island by 1841.

I have found four more children of this couple:
Catherine chr. Oct 27, 1811 Rushen
William chr. Aug 22 1813 Rushen
Richard chr. Nov 20 1814 Rushen
Margaret chr. Feb 14 1830 German.
In the 1841 census I cannot positively identify any of the children or parents, which is why I suspect they may have emigrated.
However there is a Sarah Moore aged 62 in the Onchan 1851 census Bk 2 page 12 entry 85. She again appears in 1861 age 74. She is a widow and an inmate in a "House of Industry".
In favor of Thomas 1790 and Sarah:
1. Childrens names: Margaret, after his mother
Both couples have a son named Thomas but the first son of this couple is Thomas, after his father.
Thomas and Ann have a daughter named Ann coresponding to the mother of Thomas 1788 and of course his wife.
2. Thomas (married to Ann) is aged 62 in the 1851 census, which corresponds to a birth in September 1788
In favor of Thomas 1790 and Ann:
1. His age at the time of marriage in 1809: 19 versus 18.
2. Age at death 79 on June 10 1869. If one assumes that this is the widower of Ann.
3Census data in 1861 (age 71) which corresponds to a birth in early 1790. So does the age (69) in January 1860 when he remarried.
Far from conclusive, I'm afraid
Bruce