LESS THAN STANDARD SEPARATION WHEN DEP CTLR TURNED ACR-MLG INTO ACR-MLG AT SAME ALT.

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OUR FLT DEPARTED RWY 26L IN ATL. MLG Y DEPARTED AHEAD OF US AND WE TOOK OFF NOT LONG AFTER HE WAS AIRBORNE. THERE WERE SCATTERED SMALL TRW'S IN THE AREA. DEP CTL CLRED US TO 10000'; THEN TO 14000'. WE MAINTAINED HDGS FROM 270-295 DEGS WHILE AVOIDING SEVERAL CELLS. WE OVERHEARD MLG Y; ON NORTHERLY HDGS; QUESTION HIS BASIC ROUTING AS HIS DEST WAS HOU. DEP CTL ASKED US IF WE COULD TAKE A QUICK 270 DEG HDG FROM OUR 295 DEG HDG. WE COULD NOT DUE TO A CELL DIRECTLY TO OUR LEFT; SO HE QUICKLY SENT US OVER TO ZTL FREQ. WE ENTERED SOME ROUGH CUMULUS AND REQUESTED HIGHER FROM ZTL. BECAUSE OF THE ROUGH AIR; WE HAD OUR SPD REDUCED AND WANTED HIGHER AS QUICKLY AS POSSIBLE. I ASKED ZTL TWICE FOR HIGHER WITH NO ANSWER. THEN; 30 DME FROM ATL NEAR THE 260 DEG R; WE WERE CLRED TO FL220. JUST AS WE HAD STARTED OUR CLB; WE POPPED OUT OF THE CLOUDS AND I SAW MLG Y ABOUT 2 O'CLOCK AND SLIGHTLY BELOW ON AN INTERCEPT HDG; PERPENDICULAR COURSE. BEFORE I COULD SAY OR DO ANYTHING; THE ACFT PASSED DIRECTLY UNDER OUR NOSE APPROX 200'. OUR ALT WAS APPROX 14200' IN THE CLB. MLG Y WAS AT 14000'. WE ASKED ZTL ABOUT MLG Y. HE SAID DEP CTL WAS WORKING HIM. WE COMMENTED ON HOW CLOSE HE WAS; BUT NOTHING MORE WAS SAID BY ANYONE. LATER COMS WITH ZTL BY PHONE CONFIRMED HOW VERY CLOSE WE HAD COME TO A DISASTER. THEY WERE STILL INVESTIGATING; BUT CTLR ERROR WAS INDICATED. I WOULD SAY THAT THIS WOULD BE A PRIME EXAMPLE OF WHERE AN ON BOARD COLLISION AVOIDANCE SYS WOULD HAVE BEEN A NEAR LIFESAVER; RATHER THAN THE LUCK WHICH SAVED US ON THAT DAY.