Can I separate events with period to form past perfect?
Can I say the following?
The doorbell rang. He had finished his meal.
I know I could join the sentences into one sentence to get the same meaning. I want to know if this would be correct if I write it this way? In this, the doorbell ringing comes after he finished his meal.
Correct. Nothing wrong with the way you have written it. Except I'd write "doorbell" as one word.