What was found
(CAN) SHORTLY AFTER DEPARTING NR 2 GEN WENT OFF LINE. CREW STARTED APU & THAT GEN BROUGHT ON LINE&FLT CONTINUED. COMMENCING DESCENT NR 1 GEN WENT OFF LINE & CREW DECLARED. FLT CONCLUDED W/O FURTHER INCIDENT. ON ARRIVAL WERE NO GEN FAIL INDICATION. LIGHTS ILLUMINATED. INVESTIGATION REVEALED NR 2 GENERATOR RESIDUAL VOLTAGE BELOW LIMITS & NR 1 CSD UNSTABLE AT LOWER RPM, BOTH UNITS WERE REPLACED. PREV NR 1 & 2 GCUS HAD BEEN SWAPPED FOR TROUBLE SHOOTING. NR 1 GCU RETURNED TO NR 2 POSITION & REPLACED UNIT INSTALLED IN NR 1 POS. NO GENS WERE OBSERVED TO HAVE GONE OFF LINE AT ANY TIME DURING COURSE TROUBLESHOOTING. A/C TEST FLOWN & RETURNED TO SERVICE.