Because they do work in tandem. The Bill of Rights are only applicable to the states due to the due process clause of the 14th Amendment.
Yes, the 14 amendment placed the same restrictions on State and Local Governments in respect to our constitutional rights as was once reserved for Federal Power.
But the second amendment and the equal protection clause of the fourteenth amendment are still separate legal standards. State Laws against gay marriage for example were deemed unconstitutional under the 14th amendment's equal protection. Yet there was no specific right to marriage anywhere in the bill of rights for it to work "in tandem" with.
So I'm not sure what your point is here or what it is you are trying to suggest.