I understand what you’re saying. I think you’re underestimating just how much more damage two-handed guns do than one-handed guns.
Thankfully, grim tools exists so we can use actual examples.
Let’s compare Iron Carbine and Iron Repeater (used because they’re the first common weapons in grimtools with item level 26): http://www.grimtools.com/db/items/2413 and http://www.grimtools.com/db/items/2331.
Iron Carbine fires 1.43 attacks per second, and does 40-162 damage for 101 avg damage and 144.43 avg dps.
Iron Repeater fires 1.7 times per second, and does 20-45 damage for 32.5 avg damage and 55.25 avg dps with one gun and 82.875 dual wielding.
Ranged expertise gives 28 piercing damage. With that factored in, we get:
Iron Carbine fires 1.43 attacks per second, and does 40-162 + 28 damage for 129 avg damage and 184.47 avg dps.
Iron Repeater fires 1.7 times per second, and does 20-45 + 28 damage for 60.5 avg damage and 102.85 avg dps with one gun and 154.275 dual wielding.
It’s a little bit more complicated than that because +% attack speed bonuses benefit faster weapons more than slower weapons via a formula i’m not familiar with, but it would have to make fast weapons substantially faster to tip the odds in their favor just with that flat damage alone.
Now, the more flat damage you add, the more it slides in DW pistols favor (you would need a total of 54 flat damage to move it in DW pistols favor here), but Inquisitor, at least, doesn’t provide more flat damage until Steel Resolve, Arcane Empowerment, and Aura of Conviction, where the difference in damage between 1h and 2h weapons has increased proportionally:
Steel Crossbow and Steel Sixgun (item lvl 33):
79-157 @ 1.37 attacks per second: 161.66
27-49 @ 1.7 attacks per second: 64.6 1h or 96.9 DW
Which would require 64 points of flat damage to overcome.
Assault Scorpion and Imperial Volley Gun (item lvl 58):
115-243 @ 1.37 attacks per second: 245.23
22-104 @ 1.57 attacks per second: 148.365
Which would require 98.34 points of flat damage to overcome.
Now, the Inquisitor class provides such an obscene amount of flat damage (with everything maxed out, you can get 28 Pierce (Ranged Expertise) + 32 Phys (Steel Resolve) + 36 Pierce (Arcane Empowerment) + 36 Ele (Arcane Empowerment again) + 38 Pierce (Aura of Conviction) = 102 Pierce + 32 Phys + 36 Ele) that this is actually possible, but even with everything added up it still only just starts to tip the table against 2h (assuming you don’t use that Elemental damage, and you have less than perfect armor piercing, which you do because 1h ranged weapons have crap armor piercing), and 2h weapons get the best-devotion-in-the-game Kraken.
There are other things that help push it in DW’s favor, like the WPS which fire with both weapons if you are using them both, bumping up the DW benefit from 1.5 to 2.0 some of the time (if you have them all maxed out you’ll be getting 2.0 60% of the time, making your DW bonus 1 * .2 + 2 * .8 = 1.8), but all of these things are points that if you’re using a 2h weapon, you could be using elsewhere (like the Fire Strike line, if you’re a purifier, or the primal strike line, if you’re a vindicator, or the Cadence line, if you’re a tactician).
I’m not claiming that it’s cut and dry: it’s definitely not, but you seem to be implying that dw pistols is just egregiously better than using a 2h rifle, which I think is just not true. The only inquisitor I have that’s beat Log is a DW pierce Purifier because I wanted to get that sweet, sweet flat damage + Fire Strike + WPS synergy (also, it’s the Inquisitor: every skill it has is telling you to dual wield pistols), but endgame she’ll almost definitely be nothing compared to the stupid ridiculous egergiousness that is Autentist’s 2h ranged Tactician. (Cadence is gross.)