Can’t be FORCED to testify against a spouse. Nothing to prevent one from voluntarily doing so—It’s done every day in divorce and abuse cases, for example.
You are a bit confused.
1. Testimonial privilege. A person cannot be forced to testify against a spouse because the law resepcts the absolute commitment that married people ideally should have to one another. In federal court, the person who would testify is the one who can claim the privilege. In other words, if a person wants to testify against his spouse, he can. But he cannot be compelled to do so.
2. Marital confidence privilege. A person has a right to expect that information he shares with his spouse in confidence will not be used against him. This is similar to attorney-client privilege. Nothing said in the presence of a 3rd party is privileged, and the parties have to intend the information to be confidential. However, anything said to advance a criminal conspiracy is not privileged.