FWIW, you may find some hammer guns are more amenable to having both barrels cocked simultaneously than others. I used to own a Bernardelli Italia 12 gauge that allowed me to cock both hammers after calling for the bird in skeet doubles. The hammer springs were light enough that cocking wasn't a problem, but heavy enough that there was never a failure to fire from a light blow to a primer. I had similar success with the gun on preserve pheasants over pointers, before I so foolishly sold the gun.
On the other hand, I now own a lovely 16 gauge Ferlach made (Ecker is the name on the gun) hammer gun that has much heavier springs. Even with the narrower frame and spacing between the hammers, I can't cock both simultaneously without real work. The springs are a good deal heavier than they were on the Bernardelli. There's no way I could cock both on the bird's rise in the field. I use it as a dove gun, as I'm neither moving nor (in most cases) surprised by the bird's coming into range.
My I. Hollis 10 gauge hammer gun is just a bit too wide for my hands (which are fairly small) to allow me to cock both simultaneously, although I suspect a larger set of hands could pull it off.
My suggestion to you is to try a number of different hammer guns if you want one on which you can cock both hammers simultaneously. Given some time and enough samples, you'll find one that fits you and that will allow you to do so. Doesn't solve your current problem, but if there's no special attachment to this particular hammer gun, trade it for one that will let you do what you want when you find it.
