When you dual wield you straight up double the amount of damage you can do, and with some xp investment it comes at no disadvantage. It is poor game design.
A more factual statement would be, when you dual wield if you start the round within agb in meters you double the amount of damage you can do with single handed weapons.
with the investment of between 1000 and 10000xp depending on choice of role and attributes. Other than the xp investment, and the minor loss of damage vs tough opponents, there is no disadvantage.
It is excellent game design, Dark Heresy is a brutal setting, barring some very specialist cases, offence trumps defence. What ever method you choose to deal damage will deal large amounts and whilst at a cursory glance dual wield appears to be the best method of fighting, a closer inspection shows it is far from clear.
I did further analysis of the xp costs of dual wielding, my preivious stats assumed using a point buy system, however that seems to be a best case scenario, rolling your stats, the xp cost of dual wield rises dramatically. Dual wielding master has a pre requisite of 45 agility, There is only a 1% (2.8% Hiveworld) of starting wih 40agi. Using the more experienced characters rules, this rises, to 21% (40.6% Hiveworld). Therefore it would be wise to assume that when rolling your char stats, you will require at least 2 agi advances to get to agi 45.
Furthermore, there is a 45% (15.6% Hiveworld), if using heroic characters a 10% (1.1% Hiveworld), that your char has 29 or less agility thus requiring 4 agility advances to be able to buy the final dual wield talent. And a 10%(1.1%) chance you require all 5 advances.
Factor this back into the xp requirements and how does this all change?
Desperado assassin and Warrior are okay still, they can happily take hiveworld as their homeworld, and whilst their xp costs rise due to requiring the additional agi upgrades, it is important to note, that the xp costs of everybody in the game doing anything also rise so there should be very little noticeable difference.
All of the rest will be missing the agility aptitude and in worst cases, be unable to take the hiveworld as homeworld due to lacking the perception aptitude.
The XP costs for dual wielding climb by upto 2000xp and by an average of 1000xp. Whilst the statement that you may aswell dual wield is fine if it is only costing 1500xp and you have because you have the aptitudes already. When it costs you 5000xp to dual wield in addition to the 1500xp for exprtise in WS and you now have only 3500xp left to actually use on finding the **** heretics in the first place.....
All that said couple of caveats, i forgot to factor in imperial tarot and the ability to reroll one stat into the percentages, this would have a minor effect on the overall results, slightly lessening the xp costs but further widening the gap in xp between dual wield specialists, and those splashing into it from another class.
Dual wielding is clearly superior. Because you can dual wield grenade launchers. Which is amazing.
I doubt you find a GM that allows this, the indications are that its probably not possible. At only 5kg weight the sniper rifle is not allowed to be dual wielded, grenade launchers are 12kg a piece. Whilst not strictly prohibited by the rules, it just to me seems like a no chance thing.