The question is in the title.
I have joined sentences into a large text, which I then call analyze_sentiment on. The goal is to pull sentiments for the individual sentences - exactly the ones originally joined. 
I first clean out all punctuation, lower the characters, capitalize sentences, end them with . and join with a space. 
Here is an example of two sentences that Google considers to be a single sentence.
She answered my questions with ease Thx. Tyler was so considerate.
However,
She answered my questions with ease Thx. Sam was so considerate.
works correctly.
You can try this yourself by going to their natural-language page and trying the API.
If I know the splitting conditions, I can format my original sentences accordingly.