You are incorrect. The advantage that the Commando has is that he is not hindered by the presence of the offhand weapon. The offhand weapon natively has low accuracy which combined with the mutli-attack resolution of the Merc's abilities, virtually guarantees at least one miss when attacking an enemy target. Why is that important? Because of Retaliation/Riposte. It means that when the Merc uses RapidShots vs. an attacking enemy melee, he is causing more damage to himself (the Merc) than he is to the enemy melee. That is retarded.
In order to somewhat ameliorate this negative effect, Mercs need to stack more accuracy than do Commandos, who can stack more Power. This is why the Commando is slightly superior to the Merc. By way of comparison, the dual wielding Repub class that has a single wielding mirrored Imp class (Gunslinger vs. Sniper), specifically has an ability that allows him to increase accuracy by 30% for just those situations.
Yes, well we have an energy shield thing that has low accuracy that guarantees atleast 1 miss.... I think. I'll have to check again later but I'm pretty sure it does and the 2 mirrored subclasses are the exactly the same damage wise.