I think there is some confusion here.
most companies list a set of weather/ runway requirements if you are going to be in a scenario That doesn’t need an alternate at all, but that’s the planning stage.
as alluded to above if you get to airfield xxxx , considering all factors, you can commit to land below diversion fuel, as long as you protect final reserve. Does not matter if xxxxx has one runway, two etc, Or if alternate yyyy has one or two , that’s in your decision process.
you can’t nominate another runway on the same airfieldas an alternate, it’s an alternate airfield.