Search warrant Cellphone

To search a person's cellphone you have to have a warrant. Do you have to have 2 warrants , 1 for sender of text and 1 for receiver of text or vice versa? EXAMPLE: In otherwords LE got warrant for my cousins phone. Can they look at our texts back and forth or will they have to get a warrant for my phone too? If they suspected me and him committed a robbery, why would they get warrant for just his phone and not mine? Wouldn't it be invasion of privacy or not?