PLT IN SMA ACFT W OF PDK ARPT HAS NMAC WITH TWIN CESSNA INBOUND FROM THE W LNDG AT PDK.

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IT WAS XA20 PM; JAN/WED/99. I WAS FLYING FOR A LCL COMPANY. I WAS 7 MI NW OF DEKALB-PEACHTREE ARPT FOLLOWING HWY 400 (I-19) SBOUND; ON THE E SIDE OF THE HWY. I LISTENED TO THE CURRENT ATIS-OSCAR (WHICH WAS MISSING THE CURRENT ALTIMETER SETTING). THEY WERE USING SPLIT FREQS; TFC INBOUND FROM THE W CONTACT TWR 127.2; TFC INBOUND FROM THE E CONTACT TWR ON 120.9. I CONTACTED TWR ON 127.2 AND THEY CLRED ME TO TRANSITION AT 2900 FT MSL. I PROCEEDED TO TRANSITION SBOUND; ABOUT 3 MI INTO THEIR AIRSPACE; THEY ADVISED ME OF A KING AIR DEPARTING ON A WESTERLY HEADING AT 11 O'CLOCK POS AND 2 MI WHICH I SAW; AND OTHER TFC INBOUND FOR LNDG A TWIN CESSNA AT 12:30 O'CLOCK POS AND 1 MI (THE VISIBILITY WAS VERY POOR TO THE W DUE TO THE SUN SETTING) I DID NOT SEE THE TFC. THE TWIN CESSNA WAS TALKING TO TWR ON 120.9; ACCORDING TO THE ATIS HE SHOULD HAVE BEEN ON 127.2; I CONTINUED TO LOOK FOR TFC. TWR THEN GAVE ME A TFC ALERT; 'ACFT X TFC IN YOUR IMMEDIATE VICINITY AT YOUR 3 O'CLOCK POS.' I THEN LIFTED MY R WING IN A CLBING L TURN TO GET A BETTER LOOK OUT THE R SIDE OF THE ACFT AND SAW THE TWIN CESSNA AS HE WAS IN A DSNDING L TURN AT 50 FT BELOW ME. JUST AS THE TWIN CESSNA FLEW UNDER ME TWR ADVISED ME THAT HE HAD ME IN SIGHT.