THE HELO WAS IN LEVEL CRUISE FLT AT 1500 FT AND 115 KTS WHEN THE NO. 2 ENG CHIP LIGHT ILLUMINATED, ACCOMPANIED BY LOWERING OIL PRESSURE. THE PLT REDUCED PWR AND STARTED A RT TURN TO RETURN TO BASE WHEN HALF WAY THRU THE TURN A LOUD BANG OCCURRED FOLLOWED BY COMPLETE POWER LOSS OF BOTH ENGINES AND ENG CMPT FIRE. A SUCCESSFUL AUTOROTATION ENSUED, AND THE FIRE WAS EXTINGUISHED WITH THE ACFT SYSTEM AFTER LNDG. LATER EXAMINATION REVEALED THAT THE NO. 3 BEARING HAD FAILED. ULTIMATELY RELEASING THE PWR TURBINE ASSY FROM THE ENG. THE TURBINE DISC WELDED TO THE TAILPIPE CENTER BODY, FAILING THE DISC. FRAGMENTS STRUCK THE NO. 1 ENGINE WHICH RESULTED IN ITS UNCOMMANDED SHUT DOWN. BOTH ENGINES HAD A LENGTHY HISTORY OF CHIP ALARMS. LYCOMING HAD RECENTLY APPROVED A 100 HR. EXTENSION TO THE RECOMMENDED HOT SECTION INTERVAL AFTER A REVIEW OF MAINT. RECORDS. THREE SCAVENGE CHIP ALARMS OCCURRED DRG THE 50 HRS PRIOR TO THE FAILURE.