RPTR STARTED THE WRONG ENG.

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ON JUL/XX/94 AT XA00 PM LCL; I WAS SECOND IN COMMAND OF A PART 121 REVENUE FLT FROM DFW. THE CAPT AND I WERE DISCUSSING WHETHER OR NOT WE WOULD BE WT LIMITED AT OUR DEST DUE TO HIGH TEMP AND HIGH ELEVATION. WE WERE PERFORMING OUR PREFLT DUTIES ON THE 'HARD STAND' RAMP AT DFW IN OUR 2 ENG TURBO PROP. THE ATMOSPHERE IN THE COCKPIT WAS CONFUSION AND HIGH NOISE. WE WERE RUNNING LATE FOR OUR XA00 PM SCHEDULED DEP BECAUSE THE #1 FLT ATTENDANT KEPT GETTING AN INCORRECT PAX COUNT (TOOK HER 3 ATTEMPTS: OUR DELAY WAS CREDITED TO THE FLT ATTENDANT). I WAS PERFORMING MY NORMAL FO DUTIES OF PREFLT AND WAS VERY BUSY. THE CAPT WAS ON THE TELESIS WITH DISPATCH DISCUSSING OUR DEST. COMPANY AND RAMP FREQS WERE HEARD ON THE LOUDSPEAKERS. THE FLT ATTENDANT WAS IN AND OUT OF THE COCKPIT OFTEN DURING THIS TIME. THE CAPT AND I PERFORMED THE 'BEFORE START CHKLIST.' THE RAMPER GAVE THE SIGNAL TO START #2. THE RAMPER THEN LEFT THE FRONT OF THE ACFT TO LOAD BAGS. THE CAPT THEN GOT A CALL FROM DISPATCH AND TOLD ME TO 'GO AHEAD AND START IT.' NORMAL COMPANY PROC IS FOR THE CAPT TO START BOTH ENGS. (I HAVEN'T STARTED AN ENG ALL YR.) I PUSHED THE WRONG BUTTON AND BEGAN THE START SEQUENCE FOR #1. THE RAMPER GAVE THE CUTOFF SIGNAL AND I STOPPED THE START SEQUENCE BEFORE THE SEQUENCE WAS COMPLETE. NO ONE WAS STRUCK OR HURT. IF THE RAMPER HAD BEEN ON A HEADSET AND REMAINED IN FRONT OF THE ACFT AS POLICY DICTATES; MY ERROR WOULD HAVE BEEN CAUGHT SOONER.