Ted, I think the rationale for a secondary system is the sustained high speeds on bigger tracks. On a sprint track in an FSA you are only momentarily near your top speed. At road races you are at sustained higher speeds, sometimes averaging 85mph even in a TAG.
The dual brake requirement that the WKA insists on has always a barrier to entry for traditional sprinters. We already know this.
So the debate moves to safety. Start with good tech on a single system including verifying the backup cable is indeed effective. Without that a good, working, effective tether you can have a triple circuit and still be in deep, deep doo doo.