Can the jail prevent co-defendants in jail from writing back and forth to each other?

Of course jail mail is a wonderful source of information, but sometimes it may be necessary to prevent communication between two co-defendants.

Do the inmates have a right to be able to write whoever they choose?

I've done that before. I just called the S.O. and told them that these two are co-def's and shouldn't have any contact with one another. They still get creative and find ways to do it like the time they found one of them smuggling a letter to the other in his shoe but you can stop "official" communication/contact.