DRG ARR IN HVY TRAFFIC, REEVE FLT 726 (BOEING 727) & WILBURS FLT 304 (CESSNA 402B, N969JW) RECD RADAR ADZYS FOR VISUAL APCHS TO RWYS 6L & 6R, RESPECTIVELY. SUBSEQUENTLY, BOTH FLTS WERE CLRD TO LND. AT 1555:04, THE LCL TWR CTLR TRANSMITTED THAT THE WND AT THE APCH END OF RWY 6R WAS FM 340 DEG AT 9 KTS & THE MIDFLD WND WAS 330 DEG AT 7. AT APRX 1557, FLT 726 LNDD ON RWY 6L. APRX 69 SEC LATER, AS FLT 304 WAS X-ING THE THRESHOLD (THR) OF RWY 6R AT ABOUT 100' AGL, THE ACFT ENCTRDWAKE TURBC (W/T) & ROLLED SHARPLY TO THE RGT, BYD 90 DEG. THE PLT CORRECTED WITH AILERON & PWR, BUT THE ACFT LOST ALT & HIT THE GND BFR RCVRY. THE WND DRG THE 1601 WX OBS WAS 020 DEG AT 5 KTS. RWY CTRLNS WERE 700' APART, THRESHOLD OF RWY 6LWAS 4600' BYD THE THRESHOLD OF RWY 6R. THE APCH CTLR DID NOT ADZ EITHER FLT OF THE OTHER'S PSN, NOR DID THE TWR CTLR GIVE A W/T ADZRY TO FLT 304. CALCULATIONS SHOWED VORTICES COULD HAVE DRIFTED FM RWY 6L TO RWY 6R IN AS LITTLE AS 28 SEC.FAA HNDBK ADDRESSED W/T ADZY FOR PARALLEL RWY OPNS WITH LESS THAN 2500' SEPN, BUT DID NOT ADDRESS OFFSET THRESHOLDS.