I've owned three hammer guns, two of which are still in my vault. One of them (a Bernardelli Italia 12 gauge) had a combination of sufficiently light springs and some particular angle to the hammer spurs that allowed both hammers to be cocked simultaneously as the gun was raised. I shot skeet doubles, low gun, with hammers cocked after I called for the bird and never had an issue.
The other two, a Charles Moore 12 gauge and a Ferlach made (I've blanked on the maker's name, but it's a nice 1920s gun) 16 gauge have heavier springs or sufficiently different angles on the hammer spurs that I cannot cock them both simultaneously as I mount the gun. I usually cock one hammer, fire the gun and then dismount the gun and cock the other hammer if I'm firing a follow-up shot. I make very, very few follow-up shots, as you might imagine.
With a smaller gun (say a 20 or a 28 gauge), you might have better luck as the space between the hammers is less, allowing better leverage with the right thumb.
Rem
