An American double would need to exceed in the following catagories:

A: light weight.
B: compact.
c: scoped.
D: rugged.

Merkel used to make a trim little 30-06 that was configured for scopes. A 45-70 would be all but useless unless you have nostalgia for it. As said before, a scoped 9.3x74R is the holy grail of doubles - you can hunt anything with it.

Alas, let's ask the undamental question, why a double? The answer remains the same as always: redundancy. You can take two rifles on a trip, but you always leave one back at camp. I like the fact that you can take down a double and stuff in into you backpack while you hike in. It is for the aforementioned reasons that I will be taking my Merkel .375 to Colorado elk hunting this year as my primary rifle.