This sentence needs one comma. Can you see where?
Hint: it’s not before and.
The man in the closet and the girl under my bed are scared, too.
A comma makes the difference between too meaning excessively or meaning also/as well.
This is, too. It isn’t a terribly informative sentence on its own, but it is a complete sentence, and in context, the reader would know what noun the pronoun this refers to: This is one of my favorite books. This is, too.
This is too. It is not a complete sentence. This is too… what? Too difficult? Too boring? Too simplistic? Too scary? Too cute? When you leave the comma out, you’re indicating too is an adverb meaning excessively, and that adverb needs something after it to be excessive.