Dual Fmc fail....
Unable RNP....infm ATC of degraded navigation performance. Consider making a PAN if in non radar environment. Try and request radar headings. All your route prior to the failure would have been copied onto your Cdu(s).Therefore following the route would be NO PROBLEM. However any new waypoints would have to be added using Lat/Long posn. Lnav/Vnav will not be avbl,however Lnav can be re-engaged later using old route. Basically revert to using good old INS system procedures.All radios ie Adf's and Vor's will have to be manually tuned thru Cdu's for navigation. No Vref/Vapp avbl.So open the ol handy dandy and dust those performance pages...
Most importantly land back at an airfield with the closest bar...
I got a feeling you'll need the drink!