What’s the subject of this sentence?
The subject of this sentence is "that", however, it is omitted in this sentence. It is common to omit words in English when speaking, fast, casually, and when the word is already implied. In this case, "that" is implied already because Olaf thinks that both of their names are Sven, and so it will be easier to remember their names. In English, it is also common to drop subjects when the subject is "this" or "that".