I've stumbed upon this sentence in Mauilik V's answer on this site:
If you have an iPhone 6, for instance, you can use both the sentences to describe its functionality.
The use of the definite article in the bolded position seemed strange to me, and I edited the sentence to
If you have an iPhone 6, for instance, you can use both sentences to describe its functionality.
Both the girls were very proud of their achievements at school.
So, was I wrong in changing the sentence and if not, why? In which cases is it not OK to combine both and the?