Data Highway Plus is fundamentally not suited for "redundant media". There is only one DH+ port per network interface, and no simple way to build a "redundant" twinaxial cable system.
One way to get fault tolerance in DH+ media is to use a "fault tolerant fiber optic loop". The best vendor for fiber optic DH+ product is Phoenix Digital.
Data Highway Plus is fundamentally not suited for "redundant media". There is only one DH+ port per network interface, and no simple way to build a "redundant" twinaxial cable system.
What I've got to say has nothing to do with DH+ but I do remember a redundant cabling option for A-B RIO. We'd 1/2 the cable length and go node to node with 2 cables. You had to make sure that there wasn't too much difference in the lenghts of the two cables.