I understand what you are saying. The other option is Mahala was born in 1860. Census records are not always as dependable as one would want them to be. An age discrepancy of anywhere from 1 to 5 years is not uncommon.
The census reading goes from Sept. 11 which included hshd's. 967 thru 979, then to Sept. 12 which included hshd's. 980 thru 987, then back to Sept. 11 for hshd's. 988 thru 1003, and then back to Sept.12 for hshd's. 1004 thru 1017. There could most certainly be some room for discrepancies here. Also to further compound the problem on Sept. 15 there were a total of 105 readings taken which would not be possible for a man on foot and/or horseback to accomplish in this area.
Though that has nothing to do with the root of the problem you're trying to solve it just soiidifys the fact that census takers did make mistakes, oten times not even going to residences but relying on other people in the area for information pertaining to a particular household. That is one reason why one of the mosat accepted forms of information in genealogy must sometimes be taken with a grain of salt.
I also checked the Mercer County death records and there is no Mahala to be found there. These are not often accurate as in the case with birth records they were not often reported and filed.
In the 1880 summers County census in hsh. 209 you find W.R. Neely and family and in hsh. 210 you find Josephja nd Mahala living with his mother and family. Joseph's age is listed as 22 and Mahala as 19 and I assume their son Frederic 10/12 born in July.
In the 1900 Summers County census Mahala is listed as being born in June of 1861.
I thnk there were 2 mistakes made. There was only one Mahala and they were not twins and Mahala was born in 1860 and not 1861. Everything I've seen points to one Mahala and we do know she was listed in the 1860 census. Your thoughts again on this please.