Together, I can...

Together, I can beat schizophrenia.

This is a joke about mental illness. When schizophrenia is represented in popular media, they treat it as a illness which causes people to have multiple personalities. Ordinarily someone would say "Together, we can beat X" to indicate that the problem X is not insurmountable if the power of multiple people are used in attempting to defeat it.

Since you can't be together by yourself, the joke indicates that the jokester is delusional, and believes that he is in fact multiple people.