- You insulted him; this is his complaint.
- His complaint is that you insulted him.
- He complained that you insulted him.
When in doubt, you could always use a semi-colon, like in the first example, but sometimes rephrasing or rewording proves best, such as the second and third examples.
It all depends on the audience and the syntax you want to keep. Combining two sentences can take many forms, and it all depends how much of the original sentence you want to keep. Do you wanna keep the original tone or or the original subject?
For example, you can combine the sentences with a semicolon, such as: You insulted him; that was his compaint. His complaint was that you insulted him.