I assume like this:
I have constructors supplying copper wire, and they split into three belts, one feeding Cable Factory A, one feeding Cable Factory B, and the other feeding Beacon Factory.
During normal operation, Feed Cable factory A and Beacon Factory.
If Cable Factory A can no longer accept wire, feed Cable Factory B and Beacon Factory.
If Beacon Factory can no longer accept wire, feed Cable Factory A, Cable Factory B.
Never feed all 3 factories.
If Only one Factory can accept wire, send excess to storage.
Personally, I don't know how useful such a system could be. It my save real estate, or simplify logistics a bit.