Calling "/focus" with nothing after it sets your focus to your current target, so doing "/focus /cast [target=focus] spell", is functionally equivalent to just doing "/cast spell" (with the side-effect of having changed your focus).

But unless I misunderstand what you're trying to do, you don't really need to use focus. Once your second lock has done /assist, your main can switch targets and the secondary lock won't switch unless you hit assist again.