Remember that DH+ is a peer-to-peer network. While it uses the same wire, its functionality different from the master/slave RIO network.
RIO can only have one Scanner and multiple Adapters.
This separation of functions is one of the reasons Rockwell Automation developed the ControlNet network for the PLC-5 controllers, because that combines the peer-to-peer connectivity of DH+ with the I/O scanning of RIO.
Most modern ControlLogix systems use EtherNet/IP, which mixes both the services of cyclic I/O and peer-to-peer messaging on the same network.