RS485 and Serial Bus

I currently have a system that has two distribution equipments (one as backup) linked by ethernet that both transmit data one flagged live other backup to two clients. The two clients take the live data. If there is a problem with a distributer then the client accepts the backup. Currently RS422s are used in a mesh topology. We now need NMEA format which I gather does not support flagging of live and backup. How can I get round this without the introduction of a switching unit. A suggestion has been to have one distribution system transmitting and the other not using RS485s so that if a failure of one distribution system occurs then the other distribution system starts transmitting. However I am not sure of the topology that would be required for this to work. Can RS485s be used in a Serial Bus configuration where one of the distribution systems acts as a client (together with the other two) until it is needed, whereby it starts transmitting? Would I need new driver cards in the clients for RS485s?