ABOUT 40 MI FROM DESTINATION IN HEAVY RAIN, BOTH ALTERNATORS FAILED. ELECTRICAL LOAD WAS REDUCED & A RADAR MONITORED ILS APCH WAS MADE. AFTER DESCENT THROUGH THE CLOUDS INTO VMC AN ATTEMPT WAS MADE TO LOWER THE LANDING GEAR ELECTRICALLY.AT THAT TIME ALL ELECT POWER WAS LOST. THE MANUAL EMERG GEAR EXTENSION WAS USED UNTIL RESISTANCE WAS FELT. DURING THE LANDING THE GEAR RETRACTED. THE LANDING GEAR MOTOR CIRCUIT BREAKER WAS NOT PULLED PRIOR TO THE GEAR BEING CRANKED DOWN IAW THE PLT'S OPERATING HANDBOOK. CESSNA ACFT CORP HAD PUBLISHED 2 SERVICE INFO LETTERS WHICH APPLIED TO ACFT EQUIPPED WITH THE SAME ENG/ALTERNATOR INSTALLATION AS THE ACCIDENT ACFT. THESE LETTERS RESULTED FROM MOISTURE INDUCED INTERRUPTION OF ALTERNATOR OUTPUT. ADDITIONALLY, THE ENG/ALTERNATOR/COWLING INSTALLATION OF THE BE-58P IS SIMILAR TO THECESSNA ACFT AFFECTED BY THE LETTER.