I've now received the two marriage certificates referred to above, and feel there is a possibility that the two grooms might be the same person because, in spite of their occupations being different, their fathers' names match. Only the 1839 marriage lists father's occupation for either partner, though.
Where the two brides are concerned, the only thing I can tell for definite is that they weren't sisters. Mary Glover (1839 wedding) was a widow, the daughter of a collier named Joseph Spate. However, Eliza Glover (1844 wedding) was a spinster and, as I already knew, the daughter of Benjamin Glover, although she doesn't state his occupation on the wedding certificate.
The 1839 wedding took place in the parish church in Llanwenarth, ehreas the 1844 wedding took place at St George's chapel in Tredegar.
I'm now even more confused than before. Any ideas as to where I go from here?