Wouldn't a battery charging case be easier than 2 phones? Mophie has a few that have more capacity than the internal phone battery. I haven't had any problems with battery life until after two years of use with any of iPhones I've owned (8gb original, 3GS, & 4s). Still no problems with my 4s, knock on wood.
Although, I've noticed that since I got my ipad, I only use my phone when I need to call or text, or occasionally as a hotspot for my ipad. So other than when it is a hotspot, I almost never go below 50%.