I've been able to do it in the past. However (based on what I've been told by GAs & Res), with the somewhat recent changes to maximum layovers w/r/t upgrades, if there is more than 12 hours between the two flights there is a strong likelihood that you won't be able to upgrade both flights with one CR1. Then again, YMMV and you might be able to find someone to force it for you. Good luck!