Just don't forget, a "backup" bow HAS to be equal to the bow you hunt with. If you lose the first, and suddenly you have nothing but the back up, its like shooting a new bow all over again . I went to Alberta once for a whole month. Work was slow so I went up and worked for the guide for three weeks in return for the last week of hunting. I was shooting a Silvertip with another bow as backup. The guide moved the bow while I was out in the bush, and left it in the blaring sun...only 4 hours of dark at the time up there. It didn't delaminate but the limbs twisted terribly and there was no fixing it. My backup bow was shorter and shot much differently. It was brutal, I shot nonstop the first two days that I was supposed to hunt. Totally different from my Silvertip. This is an extreme situation...but nontheless it could happen to you. Make sure you are as accurate with your backup as your main bow.
And yes, usually bows do have a sweet spot depending on the draw length they are made for. Good luck!!