SWTOR formula list

[LagunaD]

There are two cases where one side of a class "mirror" dual wields, while the other uses an enhanced 1H weapon:

Commando(1H)/Mercenary (DW)

Gunslinger(DW)/Sniper(1H)

I suspect it's probably not that hard to tune the damage of an Assault Cannon or Sniper Rifle to match the damage of MH+OH blasters (including OH penalties).

Edit: To answer your specific question, all the damage coefficients for Full Auto and Unload are identical (except the BH has an offhand attack).

Unload: 169 + 0.7*MH + 1.05*RangedBonus + 0.7*OH
Full Auto: 169 + 0.7*MH + 1.05*RangedBonus

I suspect it's harder to tune the damage of a dual-wielding Marauder and a single-saber Juggernaut (using the same weapon) to be nearly the same, although in this case they can use the AC-specific abilities to compensate.

Not sure to what extent it's true of Mercenary and Gunslinger, but on my Marauder the OH weapon damage landed was about 15% of my total melee damage.