B737-800 flight crew reported dual FMC failure inflight. Failure was short lived; as FMC's reset in approximately 25 seconds.

Narrative

During cruise; about an hour after departure; both FMC computers stopped working for about 25 seconds (roughly). The FMC option disappeared from both CDU MENU screens. Both moving maps stopped; all performance data was removed and the autopilot defaulted to CWS roll (as anticipated) when the navigation data became unavailable. We began the process of verify raw data navigation when both FMC computers came back online and proceeded to operate normally. Operationally; we called Operations Control and Maintenance Control and concurred that we were safe to continue. Basic navigation was unaffected and so the safety of flight was not reduced in the event it happened again. However; it does seem to qualify as a serious impact on aircraft systems -- especially since the design redundancy did not work (both sides failed simultaneously). Because the system reset itself normally and did not repeat the fault; it qualified as an INFO-ITEM and the AML entry was made in ZZZ; but the event also appears to qualify as a mandatory report. The description is the same verbiage used in the report. There were no additional factors that indicated the reason that both FMC computers would simultaneous shutdown and then simultaneously restart. No turbulence; we were not interacting with either CDU at the time; nothing. The event is very strange since it caused a breakdown in the 'design through redundancy' principle.Simultaneous failure of redundant systems is a bigger problem than 'an anomaly'. We happened to be in cruise within VOR and VHF range. If this had happened at a more critical time of flight-- where time critical usage of the FMC is greater-- this would have been a larger problem. If this had happened when we were beyond VOR/VHF range; this would have also caused a more serious impact on flight safety. When the FMC computers went offline; we had no access to GPS or IRS navigation data. We also lost all access to FMC performance data. Finding the cause and correcting it appears to be a high priority.