7x65R Brenneke, 30R Blaser, and 9.3x74R would all be sound choices for a double that would see use in NA. These would also translate well to Africa.
Are they offering belted cartridges as an option? If so, that would certainly broaden your choices.
I've always thought a double in 348 Winchester would be very cool.
Cabela's has a 348 Win. listed cool!!!
http://www.cabelas.com/product/Josef-Winkler-Boxlock-Double-Rifle-348-Winchester/1170261.uts?Ntk=GunLibrary&searchPath=%2Fcatalog%2Fsearch%2F%3FN%3D%26No%3D20%26Ntk%3DGunLibrary%26Ntt%3Ddouble%2Brifles%26Ntx%3Dmode%252Bmatchall%26WTz_l%3DHeader%253BSearch-Gun%2BLibrary%26WTz_st%3D%26WTz_stype%3DSP%26form_state%3DsearchForm%26search%3Ddouble%2Brifles%26x%3D14%26y%3D8&Ntt=double+rifles
