First, I started off using the sawed off shotgun + a Revolver as my dual wield.
You can chop folks down @ close range with the shotty... or split their wig apart (from distance) with a Revolver.
Nice combo.
Then after awhile, I switched to using 2 Schofield pistols. This was awesome.

The reload rate was a lot faster than the Shotty+Revolver combo.
But now I'm using 2 pistols taken from 2 of the dead bodies of
Famous Gunslingers.
I like their fast reload rate, their range & the damage they do.

Plus the engraving / designs are very cool.
Jim “Boy” Calloway's Pistol
Emmet Granger's Pistol
But I might go back to using a pair of Schofield Revolvers. I got real comfortable using those.
SIDENOTE: WEAPONS TIP >> I just learned that you can configure a
'default set' of weapons you are carrying, every time you dismount your horse.
For those who do not know....
1 - Visit any Gun Shop.
2 - Select
"Customize"
3 - On the Customization screen, you can choose which (default) Weapon to carry on your back & on your shoulder, plus you can choose each pistol you want to carry, and select a different default 'melee' weapon (i.e a knife, or hatchet, or axe, or tomahawk, or whatever).
If you do this... every time you dismount your horse, you will know exactly which weapons you are carrying 
so you don't have to scramble back to pick up that shotgun, or that bow you might've left behind.
Not sure if everyone else knew this.

Sshheeeiiit.... I been playing for a whole week... and I just learned this.
Hope this helps.