There is only a single sentence starting with 'if' and not two. The phrase 'it is therefore ..' is preceded by a comma and not a period. Hence, there's a single sentence and not two as you are referring to.
The sentence has a typo: "If one PostgreSQL server instance is to house projects or users that should be separate and for the most part unaware of each other, it is therefore recommended to put them into separate databases."
The word "therefore" ("it is therefore recommended") is mistakenly included. To correct the mistake, just remove the word. Keep up your awesomeness!
I'm not sure this is a mistake. The "therefore" refers to the statement in the previous sentence, explaining *why* it is recommended.
