Paula - it was not uncommon for two or more church weddings in the same family to be celebrated on the same day. Often this was because the eldest daughter was getting married [at last] so clearing the way for her sister to marry [a traditional convention]. I think it was much less common for this to happen in registry offices as the celebrants were less likely to adhere to traditional values.
In any case, where searches are done on 1837online you are likely to see these very clearly as the record usually [not always] shows others on that page.
Also the couples often took turns to be witnesses to the signing of the register. Those are also shown on certificates.
