There is an implicit assumption that dual-capable means you get a second mission cheap. An SSBN is dual-capable as were the Tornados and V-force before that.
What is conveniently forgotten is "nuclear withhold". That part of the force held back as part of a nuclear deterrent is essentially not dual role, like an SSBN in fact.
As said earlier, that held back force becomes part of the use-it or lose-it argument, an argument more likely wit h an aircraft based deterrent.