I think I understand the problem you are having but correct me if I’m wrong.
When you import a new deck such as ‘sentences2′ it will be created as a separate deck on Anki. To solve this you drag the new deck onto the main deck that you are using. In your case you would drag the ‘sentences2′ deck on to the ‘sentences1′ deck to collect them together.
Hope this helps ^^’
p.s. On a side note re-naming the main deck that you’re using for each section can be useful when it comes to adding new decks over time.