As long as your INS and either your DME's or your GPS are working, so that all the navigational data required by your serviceable FMS is available, your navigationa performance is unaffected and you're still RNP5. If you lose the other FMS while en-route, and it is included in your aircraft's Certificate of Minimum Navigation Performance (CMNP), you must declare yourself non-RNP5 to ATC and follow instructions. Basically, you will have to exit RNP5 airspace and descend under ATC below the RNP5 flight level applicable to the state you are flying over (This varies from state to state within Europe, naturally!) In the (admittedly unusual) case where an FMS isn't needed to maintain the required accuracy using INS with GPS or DME then you can fly RNP5 as long as you have these operating. Note that you could end up non-RNP5 even with both FMS serviceable, if you lost any navigation sensors that were required by the CMNP.
This information should be included in the operations manual, in the part govening RNP5 Operations. If it isn't, you aren't approved for RNP operations regardless of the accuracy of your navigational equipment.
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Through difficulties to the cinema