A320 FLC DEV FROM ENG STARTING PROC WHILE GND CREW IS STILL LOADING BAGGAGE AFTER PUSHBACK FROM GATE AT DEN; CO.

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AFTER BEGINNING PUSHBACKED FROM GATE IN DENVER; THE MECH CLRED US TO START BOTH ENGS AND I ISSUED INSTRUCTIONS TO THE FO TO START THEM BOTH. PRIOR TO STOPPING THE PUSHBACK; MECH SAID SOMETHING ALONG THE LINES OF 'LET'S AMEND THAT -- YOU'RE CLRED TO START #1 ONLY.' I ACKNOWLEDGED THAT CLRNC BY REPEATING IT BACK TO HIM AND THEN INSTRUCTED THE FO TO START #1 ONLY. THE FO ACKNOWLEDGED THAT INSTRUCTION. AFTER STOPPING; THE PARKING BRAKE WAS SET AND I CALLED UP THE DOORS PAGE IN ANTICIPATION OF ADDITIONAL BAGS BEING ADDED TO THE CARGO PIT. DURING THIS 'DEAD TIME.' I WAS LOOKING TO THE E AT DEVELOPING WX AND I LATER LEARNED THAT ACARS WAS GIVING US FINAL WTS. IN TRYING TO STAY AHEAD OF THE AIRPLANE; THE COPLT HAD INADVERTENTLY PUT THE #2 ENG MASTER SWITCH TO 'ON.' ABOUT 30 SECONDS LATER; THE MECH YELLED; 'YOU'RE NOT STARTING #2 ARE YOU?' SIMULTANEOUSLY; I NOTICED ABOUT 26% N2 ON #2 ENG AND IMMEDIATELY PLACED THE ENG MASTER SWITCH TO 'OFF.' CONCLUSIONS: THE COPLT ACKNOWLEDGED THE INSTRUCTION TO START #1 ONLY. HE FELT THAT DOING OTHER THINGS DURING THIS TIME AND TRYING TO STAY AHEAD OF THE AIRPLANE CAUSED HIM TO GET DISTR JUST ENOUGH TO ALLOW HABIT PATTERNS TO TAKE OVER. AS A RESULT; THE #2 ENG MASTER SWITCH WAS PLACED IN THE 'ON' POS WITHOUT REALLY INTENDING TO START THE ENG. WE BOTH FEEL THAT HAD HE VERBALLY ANNOUNCED 'STARTING #2;' ONE OR BOTH OF US WOULD HAVE RECOGNIZED WHAT WAS HAPPENING AND THE START WOULD NOT HAVE OCCURRED. ADDITIONALLY; I WAS LOOKING TO THE E TOWARD WX ALONG OUR DEP PATH AND DIDN'T NOTICE THE FO PLACING THE ENG MASTER SWITCH TO 'ON.' IN THE FUTURE; I INTEND TO ISSUE INSTRUCTIONS TO START 1 ENG AT A TIME. IN CONCLUSION; WE BOTH FELT FORTUNATE NO ONE WAS HURT. HOPEFULLY; THIS OCCURRENCE WILL GET OTHERS TO THINK ABOUT THE CONSEQUENCES OF THEIR ACTIONS AND NOT ALLOW HABIT PATTERNS TO OCCUR THAT RESULT IN UNINTENDED ACTIONS.